[BUGFIX] Scheduler task aborts if LDAP returns entry without DN

Sometimes it happens that LDAP returns records with empty DN,
but this DN is required to synchronize record details from LDAP
to TYPO3.

This fix just checks if the requested record details are a valid
array, and skips this whole entry, if not.

Resolves: #60041
Change-Id: I2dc36374e8a82f174ac63907aaccc25789c0a324
Reviewed-on: https://review.typo3.org/31266
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers