Holder can be Is_Empty and Is_Universal_String simultaneously

[ivan.levashev]

I guess, that was maid intentionally, to make output variables typed in SQL, but it's counter-intuitive when reasoning about values. Maybe there should be a more explicit distinction between empty string and non-empty string? I mean, Is_Universal_String should return False for Data'Tag => League.Holders.Universal_String_Tag and Is_Empty => True unless some non-default argument was specified to Is_Universal_String.

Those who are willing to check tag of empty holder should do it in a manner distinct from everybody else who only need to deal with values and only want to see Is_Universal_String = True when there is a real string inside.

[vadim.godunko]

This is expected behavior. Holder can have 'type' but be empty. It can be useful in some cases to pass hints of expected types to modules which will return values.