[BUGFIX] Correct return type for FileReference methods

The @return annotation for the following FileReference methods is set
to "string", but these methods can also return null:

- getTitle()
- getAlternative()
- getDescription()

To avoid problems with code quality tools and to ensure type
safety the return value of these methods is always casted to a
string.

Resolves: #94144
Releases: master, 10.4
Change-Id: Id28281ca9b55f20fb02e0de3adfe510f88580141
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69155


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Richard Haeser <richard@richardhaeser.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Richard Haeser <richard@richardhaeser.com>