CSHARP-1376: Distinct should not send a null value for the query field.

Author: rstam

src/MongoDB.Driver.Legacy/MongoCollection.cs

@@ -280,7 +280,7 @@ public IEnumerable<TValue> Distinct<TValue>(DistinctArgs args)
             var valueSerializer = (IBsonSerializer<TValue>)args.ValueSerializer ?? BsonSerializer.LookupSerializer<TValue>();
             var operation = new DistinctOperation<TValue>(_collectionNamespace, valueSerializer, args.Key, GetMessageEncoderSettings())
             {
-                Filter = new BsonDocumentWrapper(args.Query),
+                Filter = args.Query == null ? null : new BsonDocumentWrapper(args.Query),
                 MaxTime = args.MaxTime,
             };