[BUGFIX] Correctly handle non-existing level in menu.directory VH

Providing a non-existing entry-level to the getPageUids() method
now returns an empty array instead of an array with key zero and
a null value.

This way we can correctly stop processing in the viewhelpers
using the MenuViewHelperTrait.

Resolves: #82507
Releases: 7.6
Change-Id: I99a610e612545648d7637e4a441dcdd1b1e51adc
Reviewed-on: https://review.typo3.org/54167


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Tested-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>