[BUGFIX] Fix fallback mechanism for module menu

If the startModule is not available the fallback mechanism
doesn't work. The check returns always true. Use jQuery to
check the length of the submodule.

Resolves: #61562
Releases: 6.3
Change-Id: I4b163ef6df86e9603da4ef50e2fb5a94193b31fb
Reviewed-on: http://review.typo3.org/32744
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>