[BUGFIX] Fix PHP type error in f:form.select.optgroup view helper

Using view helper `f:form.select.optgroup` with argument `disabled`
throws PHP type error, since array is expected, but string given.

Resolves: #92656
Releases: master, 10.4, 9.5
Change-Id: I7f1fd3a1f81c7befb15e4657c9e59ff862e663e1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66242


Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Daniel Haupt <mail@danielhaupt.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Daniel Haupt <mail@danielhaupt.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>

typo3/sysext/fluid/Classes/ViewHelpers/Form/Select/OptgroupViewHelper.php

@@ -46,7 +46,7 @@ class OptgroupViewHelper extends AbstractFormFieldViewHelper
     public function render()
     {
         if ($this->arguments['disabled']) {
-            $this->tag->addAttributes('disabled', 'disabled');
+            $this->tag->addAttribute('disabled', 'disabled');
         } else {
             $this->tag->removeAttribute('disabled');
         }