[BUGFIX] Wrong result on empty string globalString condition

A TypoScript condition like
"[globalString = GP:anEmptyGetPostVarKey = ]" returns a false
value instead of the expected true value.

Fixes: #45183
Releases: 4.5,6.0, 6.1, 6.2
Change-Id: I114b702f4b5ae5f68236874325e82974a8ba6107
Reviewed-on: https://review.typo3.org/21979
Tested-by: Philipp Gampe
Reviewed-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind