Since the rootline refactoring a test case fails. Solution is to only
call the rootline in backend mode if page id is not 0. This additionally
saves some cycles in backend context.

Change-Id: I224ae2655b06dfb1f239b06d6ac2cbb89206a7a8
Resolves: #39997
Related: #39352
Releases: 6.0
Reviewed-on: http://review.typo3.org/13966
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn