[BUGFIX] Indexing an empty string for fieldtype _intS is not allowed by solr

Change-Id: I112f06a24aec1c8f8a12459b96a6c5cbf0aad5ed
Related: #56464
Reviewed-on: https://review.typo3.org/27969
Reviewed-by: Ingo Renner
Tested-by: Ingo Renner