[CLEANUP] Fix CGL with NULL value

Commit d7580828 wrongly introduced
'null' value instead of 'NULL' as stated in our coding guidelines.

Change-Id: I94a96bca16a714bdaa1a5967ac510d8ac5c6154d
Related: #28297
Releases: 4.6
Reviewed-on: http://review.typo3.org/6253
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert