fix(document-builder): do not include null relationships in compound doc

Author: jaredcnance

src/JsonApiDotNetCore/Builders/DocumentBuilder.cs

@@ -169,14 +169,21 @@ private List<DocumentData> _getIncludedEntities(ContextEntity contextEntity, IId
 
                 if (navigationEntity is IEnumerable)
                     foreach (var includedEntity in (IEnumerable)navigationEntity)
-                        included.Add(_getIncludedEntity((IIdentifiable)includedEntity));
+                        _addIncludedEntity(included, (IIdentifiable)includedEntity);
                 else
-                    included.Add(_getIncludedEntity((IIdentifiable)navigationEntity));
+                    _addIncludedEntity(included, (IIdentifiable)navigationEntity);
             });
 
             return included;
         }
 
+        private void _addIncludedEntity(List<DocumentData> entities, IIdentifiable entity)
+        {
+            var includedEntity = _getIncludedEntity(entity);
+            if(includedEntity != null)
+                entities.Add(includedEntity);
+        }
+
         private DocumentData _getIncludedEntity(IIdentifiable entity)
         {
             if(entity == null) return null;