[BUGFIX] User settings do not obey setup.override

Add a missing isset() check whether a setting has been overridden
by user TSconfig.

Resolves: #57690
Releases: 6.2, 6.1
Change-Id: Id1290b2af85061051ce0cedff4a0be96a91c4dce
Reviewed-on: https://review.typo3.org/29239
Reviewed-by: Renzo Bauen
Tested-by: Renzo Bauen
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind

typo3/sysext/setup/Classes/Controller/SetupModuleController.php

@@ -486,7 +486,7 @@ class SetupModuleController {
 			if ($style) {
 				$more .= ' style="' . $style . '"';
 			}
-			if ($this->overrideConf[$fieldName]) {
+			if (isset($this->overrideConf[$fieldName])) {
 				$more .= ' disabled="disabled"';
 			}
 			$value = $config['table'] == 'be_users' ? $GLOBALS['BE_USER']->user[$fieldName] : $GLOBALS['BE_USER']->uc[$fieldName];