[BUGFIX] Preview links are shown for deleted pages

Preview links should not be shown in the backend, if pages are
deleted in a workspace. Currently both page tree context menu
and workspaces module offer a preview for deleted pages which
results in a "broken rootline" error once being clicked.

Resolves: #59142
Releases: 6.2
Change-Id: Id398ace2deb7dc6f000c7960d23f940b694d6229