[BUGFIX] Set correct default value for page action callbacks

This patch sets the correct default value for page action callbacks passed
for `TYPO3/CMS/Backend/PageActions`. Empty strings are not treated
properly, also the argument of `PageRenderer::loadRequireJsModule` sets
the default value to null.

Resolves: #88649
Related: #88532
Releases: master, 9.5
Change-Id: Ic5ce5eca5c0f2845ff9453e5f8d50de7cd650356
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61409


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <noreply@typo3.com>