add check for when BsonElement has no args, aka element name is null

Author: kyra-rk

src/MongoDB.Bson/Serialization/Conventions/NamedIdMemberConvention.cs

@@ -122,7 +122,7 @@ private bool IsValidIdMember(BsonClassMap classMap, MemberInfo member)
             }
 
             var bsonElement = member.GetCustomAttribute<BsonElementAttribute>();
-            if (bsonElement != null && bsonElement.ElementName != "_id")
+            if (bsonElement != null && !string.IsNullOrEmpty(bsonElement.ElementName) && bsonElement.ElementName != "_id")
             {
                 return false;
             }