[BUGFIX] Correct handling of empty Backend Layouts

Since the default value of the page properties backend_layout and
backend_layout_next_level is an empty string the value in the select
item is changed from zero to an empty string for consistency.

Additionally the BackendLayoutView will treat an empty string equally
as a zero namely as a non set Backend Layout.

Resolves: #52596
Releases: 6.2
Change-Id: Ic792f748d005f5749f24a3e979b304c23320c8fa
Reviewed-on: https://review.typo3.org/24882
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer