[BUGFIX] Add type cast for potential mountpoint information

Avoid exceptions by adding a type cast to string for the mount point
information. An additional early return avoids additional calls to
`\TYPO3\CMS\Core\Utility\GeneralUtility::trimExplode()`.

Resolves: #99946
Releases: main, 11.5
Change-Id: I18ea15c0f3e2c884e3333e55877e0f6dbe56c859
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77860
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Jochen <rothjochen@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Jochen <rothjochen@gmail.com>

typo3/sysext/core/Classes/Routing/PageSlugCandidateProvider.php

@@ -396,7 +396,7 @@ protected function findPageCandidatesOfMountPoint(
             $rootLine = GeneralUtility::makeInstance(
                 RootlineUtility::class,
                 $pageCandidate['uid'],
-                $pageCandidate['MPvar'],
+                (string)$pageCandidate['MPvar'],
                 $this->context
             )->get();
             foreach ($rootLine as $pageInRootLine) {

typo3/sysext/core/Classes/Utility/RootlineUtility.php

@@ -373,6 +373,10 @@ protected function processMountedPage(array $mountedPageData, array $mountPointP
      */
     protected function sanitizeMountPointParameter(string $mountPointParameter): string
     {
+        $mountPointParameter = trim($mountPointParameter);
+        if ($mountPointParameter === '') {
+            return '';
+        }
         $mountPoints = GeneralUtility::trimExplode(',', $mountPointParameter);
         foreach ($mountPoints as $key => $mP) {
             // If MP has incorrect format, discard it