[BUGFIX] Adjust response usage for `file/exists` route

With #83267 a JSON response object was introduced. The patch has the side
effect that any response is now an array but some usages in JavaScript
were not adjusted.

This patch does two things:

- `FileController::fileExistsInFolderAction()` does now either return an
  array for falsy results OR the file object.
- The JavaScript code is adjusted to check for a valid file object by
  requiring the `uid` property instead.

Resolves: #83568
Related: #83267
Releases: master
Change-Id: I9878f5f70b8af3ba8ec151a6ed514780547ec1ea
Reviewed-on: https://review.typo3.org/55371


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>