[BUGFIX] Check if file exists for backend logo

[BUGFIX] Check if file exists for backend logo

If an invalid file path is given, an exception is thrown if trying to
access its properties and the backend is not accessible any more.
Check if file exists before accessing. If not use default.

Resolves #84791
Releases: master, 8.7

Change-Id: Ide4c58dd03cbad1ed06ad66f558d170cd2170482
Reviewed-on: https://review.typo3.org/56731
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Kay Strobach <typo3@kay-strobach.de>
Tested-by: Kay Strobach <typo3@kay-strobach.de>
Reviewed-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Tested-by: Mathias Schreiber <mathias.schreiber@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>