Attach empty property groups instead of null for missing page info no…

…de and empty node:

RevisionsPropertyGroup, LanguageLinksPropertyGroup, GeoCoordinatesPropertyGroup
Corresponding pageinfo node won't exist at all for pages without categories / langlinks etc.
We don't want consumers to bump into NRE in this case.

WikiClientLibrary/Pages/Queries/Properties/GeoCoordinatesPropertyProvider.cs

@@ -81,8 +81,7 @@ public class GeoCoordinatesPropertyGroup : WikiPagePropertyGroup
 
     internal static GeoCoordinatesPropertyGroup? Create(JsonArray? jcoordinates)
     {
-        if (jcoordinates == null) return null;
-        if (jcoordinates.Count == 0) return empty;
+        if (jcoordinates == null || jcoordinates.Count == 0) return empty;
         return new GeoCoordinatesPropertyGroup(jcoordinates);
     }
 

WikiClientLibrary/Pages/Queries/Properties/LanguageLinksPropertyProvider.cs

@@ -150,11 +150,10 @@ public class LanguageLinksPropertyGroup : WikiPagePropertyGroup
 
     private static readonly LanguageLinksPropertyGroup Empty = new LanguageLinksPropertyGroup(Array.Empty<LanguageLinkInfo>());
 
-    internal static LanguageLinksPropertyGroup? Create(JsonObject jpage)
+    internal static LanguageLinksPropertyGroup Create(JsonObject jpage)
     {
         var jlangLinks = jpage["langlinks"];
-        if (jlangLinks == null) return null;
-        if (jpage.Count == 0) return Empty;
+        if (jlangLinks == null || jpage.Count == 0) return Empty;
         var langLinks = jlangLinks.Deserialize<IReadOnlyCollection<LanguageLinkInfo>>(MediaWikiHelper.WikiJsonSerializerOptions);
         return new LanguageLinksPropertyGroup(langLinks!);
     }

WikiClientLibrary/Pages/Queries/Properties/RevisionsPropertyProvider.cs

@@ -74,15 +74,14 @@ public override int GetMaxPaginationSize(MediaWikiVersion version, bool apiHighL
 public class RevisionsPropertyGroup : WikiPagePropertyGroup
 {
 
-    private static readonly RevisionsPropertyGroup Empty = new RevisionsPropertyGroup();
+    private static readonly RevisionsPropertyGroup Empty = new ();
 
     private Revision[] _Revisions;
 
     internal static RevisionsPropertyGroup? Create(JsonObject jpage)
     {
         var jrevisions = jpage["revisions"]?.AsArray();
-        if (jrevisions == null) return null;
-        if (jrevisions.Count == 0) return Empty;
+        if (jrevisions == null || jrevisions.Count == 0) return Empty;
         var stub = MediaWikiHelper.PageStubFromJson(jpage);
         return new RevisionsPropertyGroup(stub, jrevisions);
     }