Drop null elements in sources.json

app/Niv.hs

@@ -91,7 +91,19 @@ newtype PackageSpec = PackageSpec { unPackageSpec :: Aeson.Object }
 
 -- | Simply discards the 'Freedom'
 attrsToSpec :: Attrs -> PackageSpec
-attrsToSpec = PackageSpec . fmap snd
+attrsToSpec = PackageSpec . dropNulls . fmap snd
+  where
+    dropNulls
+      :: HMS.HashMap T.Text Aeson.Value
+      -> HMS.HashMap T.Text Aeson.Value
+    dropNulls = HMS.mapMaybe $ \case
+      x@Aeson.Object{} -> Just x
+      x@Aeson.Array{} -> Just x
+      x@Aeson.String{} -> Just x
+      x@Aeson.Number{} -> Just x
+      x@Aeson.Bool{} -> Just x
+      Aeson.Null -> Nothing
+
 
 parsePackageSpec :: Opts.Parser PackageSpec
 parsePackageSpec =

tests/expected/niv-init.json

@@ -1,6 +1,5 @@
 {
     "nixpkgs": {
-        "homepage": null,
         "url": "http://localhost:3333/NixOS/nixpkgs-channels/archive/571b40d3f50466d3e91c1e609d372de96d782793.tar.gz",
         "owner": "NixOS",
         "branch": "nixos-18.09",