[BUGFIX] Speed up BackendUtility->explodeSoftRefParserList

[BUGFIX] Speed up BackendUtility->explodeSoftRefParserList

Due to improper checking for an empty array in
BackendUtility->explodeSoftRefParserList multiple trimExplode-calls per
record are executed. Fixing this check leads to an early return and
speeds up bulk inserting.

Resolves: #63674
Releases: master, 6.2
Change-Id: Id8969c33d8ebb109018159ada6a277ab5bfdd13c
Reviewed-on: http://review.typo3.org/35187
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>