Allow nulls when expecting object or array in response (#440)

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## Description

Fix a bug which was causing an internal error when `null` was returned
by NDC for a field of array or object type.

### Product
_(Select all products this will be available in)_
- [x] community-edition
- [x] cloud
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### Type
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- [ ] highlight
- [ ] enhancement
- [x] bugfix
- [ ] behaviour-change
- [ ] performance-enhancement
- [ ] security-fix
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### Changelog entry
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Fix a bug which was causing an internal error when `null` was returned
by NDC for a field of array or object type.

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<!-- changelog : end : DO NOT REMOVE -->

V3_GIT_ORIGIN_REV_ID: 5c935ccd6720b5e5966dfa87c2e21dbb7a2b36f2

v3/crates/custom-connector/data/institutions.json

@@ -1,2 +1,3 @@
 { "id": 1, "name": "Queen Mary University of London", "location": { "city": "London", "country": "UK", "campuses": ["Mile End", "Whitechapel", "Charterhouse Square", "West Smithfield"] }, "staff": [ { "first_name": "Peter", "last_name": "Landin", "specialities": ["Computer Science", "Education"], "favourite_artist_id": 1 } ], "departments": ["Humanities and Social Sciences", "Science and Engineering", "Medicine and Dentistry"] }
-{ "id": 2, "name": "Chalmers University of Technology", "location": { "city": "Gothenburg", "country": "Sweden", "campuses": ["Johanneberg", "Lindholmen"] }, "staff": [ { "first_name": "John", "last_name": "Hughes", "specialities": ["Computer Science", "Functional Programming", "Software Testing"], "favourite_artist_id": 2 }, { "first_name": "Koen", "last_name": "Claessen", "specialities": ["Computer Science", "Functional Programming", "Automated Reasoning"], "favourite_artist_id": 3 } ], "departments": ["Architecture and Civil Engineering", "Computer Science and Engineering", "Electrical Engineering", "Physics", "Industrial and Materials Science"] }
+{ "id": 2, "name": "Chalmers University of Technology", "location": { "city": "Gothenburg", "country": "Sweden", "campuses": ["Johanneberg", "Lindholmen"] }, "staff": [ { "first_name": "John", "last_name": "Hughes", "specialities": ["Computer Science", "Functional Programming", "Software Testing"], "favourite_artist_id": 2 }, { "first_name": "Koen", "last_name": "Claessen", "specialities": ["Computer Science", "Functional Programming", "Automated Reasoning"], "favourite_artist_id": 3 } ], "departments": ["Architecture and Civil Engineering", "Computer Science and Engineering", "Electrical Engineering", "Physics", "Industrial and Materials Science"] }
+{ "id": 3, "name": "University of Nowhere", "location": null, "staff": null, "departments": ["nothing", null]}

v3/crates/engine/src/execute/process_response.rs

@@ -246,7 +246,9 @@ pub fn process_field_selection_as_list(
     value: json::Value,
     selection_set: &normalized_ast::SelectionSet<'_, GDS>,
 ) -> Result<json::Value, error::Error> {
-    if selection_set.fields.is_empty() {
+    if selection_set.fields.is_empty() || value.is_null() {
+        // If selection set is empty we return the whole value without further processing.
+        // If the value is null we have nothing to process so we return null.
         Ok(value)
     } else {
         let rows: Vec<IndexMap<String, ndc_models::RowFieldValue>> = json::from_value(value)?;
@@ -262,7 +264,9 @@ pub fn process_field_selection_as_object(
     value: json::Value,
     selection_set: &normalized_ast::SelectionSet<'_, GDS>,
 ) -> Result<json::Value, error::Error> {
-    if selection_set.fields.is_empty() {
+    if selection_set.fields.is_empty() || value.is_null() {
+        // If selection set is empty we return the whole value without further processing.
+        // If the value is null we have nothing to process so we return null.
         Ok(value)
     } else {
         let row: IndexMap<String, ndc_models::RowFieldValue> = json::from_value(value)?;

v3/crates/engine/src/execute/remote_joins/collect.rs

@@ -198,6 +198,7 @@ fn rows_from_row_field_value(
                     serde_json::Value::Object(_) => {
                         serde_json::from_value(this.0).ok().map(|v| vec![v])
                     }
+                    serde_json::Value::Null => Some(vec![]),
                     _ => None,
                 }
             }

v3/crates/engine/tests/execute/models/select_many/nested_select/expected.json

@@ -80,6 +80,17 @@
           "Physics",
           "Industrial and Materials Science"
         ]
+      },
+      {
+        "id": 3,
+        "location": null,
+        "location_country": null,
+        "staff": null,
+        "staff_first_name": null,
+        "departments": [
+          "nothing",
+          null
+        ]
       }
     ]
   }

v3/crates/engine/tests/execute/models/select_many/nested_select/no_explicit_type_mapping/expected.json

@@ -80,6 +80,17 @@
           "Physics",
           "Industrial and Materials Science"
         ]
+      },
+      {
+        "id": 3,
+        "location": null,
+        "location_country": null,
+        "staff": null,
+        "staff_first_name": null,
+        "departments": [
+          "nothing",
+          null
+        ]
       }
     ]
   }

v3/crates/engine/tests/execute/remote_relationships/from_nested/expected.json

@@ -33,6 +33,9 @@
               }
             }
           ]
+        },
+        {
+          "staff": null
         }
       ]
     }
@@ -71,6 +74,9 @@
               }
             }
           ]
+        },
+        {
+          "staff": null
         }
       ]
     }
@@ -109,6 +115,9 @@
               }
             }
           ]
+        },
+        {
+          "staff": null
         }
       ]
     }
@@ -147,6 +156,9 @@
               }
             }
           ]
+        },
+        {
+          "staff": null
         }
       ]
     }