[BUGFIX] Remove useless string cast of idn_to_ascii()

The function `idn_to_ascii()` will return either `false` or a string,
the former one is returned on errors. In #93900, the result was cast to
string, making the check for `false` useless.

As `idn_to_ascii()` can return both, the string cast is removed to make
the check for `false` functional again.

Resolves: #96399
Related: #93900
Releases: main, 11.5
Change-Id: Ib5327766fc545a75feb7389d18358aa85b76e5db
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72740


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Bartsch <bo@cedev.de>