[BUGFIX] Filter empty filelist actions after dispatching ProcessFileListActionsEvent

When listing the contents of the folder "form_definitions",
`ProcessFileListActionsEventListener` modifies the available actions in
that directory by setting any disallowed action to `null`. With #99861,
a necessary null check was removed, leading to PHP warnings.

To solve the issue, `ProcessFileListActionsEventListener` now unsets
disallowed actions. Additionally, the `array_filter()` call to remove
empty actions is now executed after handling the
`ProcessFileListActionsEvent` event.

Resolves: #99915
Related: #99861
Releases: main
Change-Id: Ia04ac7f3d6db3eb986e6eaad3e3d41f8a6d54bec
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77821


Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Thomas Hohn
Tested-by: Benjamin Kott <benjamin.kott@outlook.com>