[BUGFIX] Module Menu throws PHP warning for top level menu items

When rendering the module menu icons, the menu items
on the top level (Web, File, User, Admin) don't have an icon,
however the getModuleIcon functionality processes them,
and checks for getimagesize() even though no image is added.

To overcome this, the function needs to be modified slightly to
only call getimagesize if there is actually an icon.

Releases: 6.2, 6.1
Resolves: #57179
Change-Id: Icb45d53a3a61d308b5ff56612f1b9b9b54245a53
Reviewed-on: https://review.typo3.org/28639
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers