use STRINGNULL tests where appropriate in bind.c
use STRINGNULL tests where appropriate in bind.c

Parrot has a symbol STRINGNULL used as a null string value, much like
PMCNULL for pmcs. In the past it didn't get a lot of use, most null
strings simply being set to NULL. I have recently created a branch in
parrot to use STRINGNULL more extensively. My changes cause problems
in C code that erroneously assumes null parrot strings are NULL, such
as rakudo's binder.c.

Closes RT #74272.

Signed-Off-By: Moritz Lenz <2b469823646c02e9f01a7ad5b893ceac76b06bbe@faui2k3.org>

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