[BUGFIX] Early return in checkValueForInternalReferences

If an empty field is passed to checkValueForInternalReferences
in the DataHandler class - there is no need to process the
field since it's not a "real" DB field.

An early return ensures that the method is not executed
unnecessarily and for Flexforms.

Resolves: #81627
Releases: master, 8.7
Change-Id: I1143e586d80faf8d45acb54d5937175950c8e403
Reviewed-on: https://review.typo3.org/53272


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christer V <cvi@systime.dk>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Kasper Ligaard <kasperligaard+typo3.org@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Kasper Ligaard <kasperligaard+typo3.org@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>