[BUGFIX] Category Update-Wizard fails if description is null

Resolves a bug in the update-wizard which fails if the existing description-field is null. Replaces null-valued descriptions in the update-process to empty values.

Change-Id: I879360f8861197e698f50187451217d0ddc93dc5
Resolves: #62299
Reviewed-on: http://review.typo3.org/33350
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>