[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