[BUGFIX] Activating NULL value field does not work with blank string

Activating a field that supports NULL values and just using a
blank string ("") does not work. The problem is a strcmp() call
that returns a false-positive on comparing NULL to blank strings

Change-Id: I59417f5f5cd814db15e2b6b725f1778d098014f6
Fixes: #43139
Releases: 6.0
Reviewed-on: http://review.typo3.org/16599
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader