[BUGFIX] Do not add empty alert fieldChangeFunc to link browser URL

Adding an empty alert message to the fieldChangeFunc array causes
the fieldChangeFunc_hash to mismatch when pagination or search
is used in the link browser popup.

Add a check for an empty value, which is already present in another
place in Core.

Resolves: #75743
Releases: master, 7.6
Change-Id: I1a15400960d577c1a2a0a905643b56533d957100
Reviewed-on: https://review.typo3.org/50586


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>