[BUGFIX] AllowLanguageSynchronization processes null values twice

TCA columns having 'l10n_mode' defined to either 'exclude' or
'mergeIfNotBlank' (later is autotatically migrated to the according
'allowLanguageSynchronization' behavior) are processed twice on handling
modifications in DataHandler. In a result an exeception is throws which
prevents recursions on chained translations.

Checking null values with plain isset() is replaced with a new method
using array_key_exists() to consider null values as well.

Change-Id: I3f1b0cdf3f62845f3bae3632f6aaa9b2dfdc6e0b
Resolves: #80338
Releases: master
Reviewed-on: https://review.typo3.org/52091


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>