[BUG]Cannot serialize empty containers.

[EnderTheCoder]

I was using oatpp-postgresql to insert contrainers(mainly UnorderedSet) into a table as a field and my program reported this:

This error shows that oatpp-postgresql cannot serialize a common UnderedSet<> object. After diving into the source code I found that this module was using the first element from the containter to get type info of it. When trying to insert an empty container into the db, it will throw an error tells that cannot get type oid of Void type.
cause
Any empty container serialization could break the QUERY() execution.

[EnderTheCoder]

Well, I dont see any way can fix this quickly. Maybe add a more specific and helpful error msg tells that this is not implemented would be better? The current error msg is kinda confusing.