[BUGFIX] Uncaught exception if editor has no file mount

If an editor has got no file mounts, an uncaught exception
is shown in the element browser.

Fix this by checking if there is a selected folder at all.

Resolves: #52969
Releases: 6.2
Change-Id: I5f9e8cc7994edd69f6db6ae1cc647ee31e4930c6
Reviewed-on: https://review.typo3.org/25357
Reviewed-by: Stefan Neufeind
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter