[BUGFIX] Removed empty defaults for backend_layouts

The new default value has to be 0 due to the backend layout
configuration we introduced with data providers. So replacing
the default 0 with an empty string must be avoided.

Resolves: #53269
Releases: 6.2
Change-Id: Ieca1a836cba5abe261c0a099e0beb9fcab0ff462
Reviewed-on: https://review.typo3.org/25134
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer