[BUGFIX] Fix empty globalString LIT condition comparison

An empty "LIT:" condition results in NULL, so the strict compare
doesn't match with the empty string.

Solution is to cast the haystack to string before, like it was
done anyway later in the preg_match.

Releases: 6.3, 6.2, 6.1
Resolves: #59344
Change-Id: I1e4b00e454d921eedd0c03a1767d4ec57c321bd7
Reviewed-on: https://review.typo3.org/30559
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Marc Bastian Heinrichs
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters