[TASK] Use file_exists() instead of is_file()

is_file() checks whether a resource exists and whether it
is actually a file. There are several parts in the core,
where it is sufficient to check for the existence only,
which is faster.

Resolves: #55052
Releases: 6.2
Change-Id: I57b6d2f68dd3c1c45b603c3f194cc0c53c48e28c
Reviewed-on: https://review.typo3.org/26872
Reviewed-by: Wouter Wolters
Reviewed-by: Tom Ruether
Tested-by: Tom Ruether
Reviewed-by: Oliver Klee
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack