[BUGFIX] Trim email address in EmailFinisher

The ext:form EmailFinisher allows to retreive
recipients via different ways. Next to the
default yaml configuration is it also possible
to set recipients via flexform or via TypoScript.

In such cases there might be additional whitespaces
added, which would lead to the function not returning
the recipients address since GU:validEmail() would
fail. Since recipients are mandatory, this leads
to an exception in the finisher.

To prevent such "false positives", the address is
now trimmed before being checked.

Resolves: #104198
Releases: main, 12.4
Change-Id: Ia66cc73d477507aee6c954995d6afd6a771e6354
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84841


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>