Return null for interfaces in DeriveTypeFromTypeInfo (#228)

Author: Shane32

src/GraphQL.Conventions/Adapters/GraphTypeAdapter.cs

@@ -73,7 +73,7 @@ private object DeriveTypeFromTypeInfo(Type type)
  {
  return graphType;
  }
- return Activator.CreateInstance(type);
+ return type.IsInterface ? null : Activator.CreateInstance(type);
  }
 
  private IObjectGraphType DeriveOperationType(GraphTypeInfo typeInfo) =>