[BUGFIX] Handle getFileContents in LocalDriver better

The method getFileContents has return value string. But since
there is no guarantee that the file exists before calling
file_get_contents, a call to is_readable has been added.
This handles both the PHP Warning and ensures type correctness.

Resolves: #99926
Releases: main, 11.5
Change-Id: I9786a6737d7e3b0c3d04272364d868fd4101307c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77978


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: JAKOTA Design Group GmbH <suchowski@jakota.de>