[BUGFIX] Guard FAL API when called with invalid identifiers

When calling ResourceFactory::getInstance()
->getFileObjectFromCombinedIdentifier($identifier)
with $identifier === NULL or $identifier === ''
an index entry is written for storage 0 which contains the
last path segment of PATH_site as identifier.

Guard the API in two places by throwing exceptions
when an invalid file identifier is given.

Resolves: #59295
Releases: 6.2
Change-Id: Ie2d6d811193ee21b865161984ddbac240388573b
Reviewed-on: https://review.typo3.org/30526
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter