Well ... yes, they will. But this is how Oracle works and we can't change that, really, so it's irrelevant in a sense. The empty string is NULL in Oracle so the suggestion to use undefined Tcl vars as a flag and the empty string as the empty string would introduce differences in behavior between the Oracle and PG version.
That would be a maintenance headache.
Also using undefined Tcl vars as a flag for NULL would, in itself, be very error prone. A typo would lead to silent failure, not good.
