properly fix action=wezterm.action.ExtendSelectionToMouseCursor(nil)

ee5206db50 attempted to fix this by
replacing all DynValue::Null values with a special null userdata.

However, a consequence of that is that it broke:

```
window:get_config_overrides() or {}
```

because a userdata is always true, even if it represents a null value.

This commit fixes that case by being selective with the userdata usage:
we only use it when converting the value portion of a table key/value
pair.

refs: #2177
refs: #2200

luahelper/src/lib.rs

@@ -58,12 +58,30 @@ macro_rules! impl_lua_conversion_dynamic {
 pub fn dynamic_to_lua_value<'lua>(
     lua: &'lua mlua::Lua,
     value: DynValue,
+) -> mlua::Result<mlua::Value> {
+    dynamic_to_lua_value_impl(lua, value, false)
+}
+
+pub fn dynamic_to_lua_table_value<'lua>(
+    lua: &'lua mlua::Lua,
+    value: DynValue,
+) -> mlua::Result<mlua::Value> {
+    dynamic_to_lua_value_impl(lua, value, true)
+}
+
+fn dynamic_to_lua_value_impl<'lua>(
+    lua: &'lua mlua::Lua,
+    value: DynValue,
+    is_table_value: bool,
 ) -> mlua::Result<mlua::Value> {
     Ok(match value {
         // Use a special userdata as a proxy for Null, because if we are a value
         // and we use Nil then the key is implicitly deleted and that changes
         // the representation of the data in unexpected ways
-        DynValue::Null => LuaValue::LightUserData(mlua::LightUserData(std::ptr::null_mut())),
+        DynValue::Null if is_table_value => {
+            LuaValue::LightUserData(mlua::LightUserData(std::ptr::null_mut()))
+        }
+        DynValue::Null => LuaValue::Nil,
         DynValue::Bool(b) => LuaValue::Boolean(b),
         DynValue::String(s) => s.to_lua(lua)?,
         DynValue::U64(u) => u.to_lua(lua)?,
@@ -81,7 +99,7 @@ pub fn dynamic_to_lua_value<'lua>(
             for (key, value) in object.into_iter() {
                 table.set(
                     dynamic_to_lua_value(lua, key)?,
-                    dynamic_to_lua_value(lua, value)?,
+                    dynamic_to_lua_table_value(lua, value)?,
                 )?;
             }
             LuaValue::Table(table)