Describe the bug

If indexing is disabled for the mount page, the attempt to fetch solr site in ApacheSolrForTypo3\Solr\Domain\Index\Queue\RecordMonitor\Helper::addPageToMountingSiteIndexQueue will return null and there will be an exception. A check is missing there.

Code is at https://github.com/TYPO3-Solr/ext-solr/blob/main/Classes/Domain/Index/Queue/RecordMonitor/Helper/MountPagesUpdater.php#L87

    protected function addPageToMountingSiteIndexQueue(int $mountedPageId, array $mountProperties): void
    {
        $siteRepository = GeneralUtility::makeInstance(SiteRepository::class);
        $mountingSite = $siteRepository->getSiteByPageId($mountProperties['mountPageDestination']);

        /** @var Page $pageInitializer */
        $pageInitializer = GeneralUtility::makeInstance(Page::class);
        $pageInitializer->setSite($mountingSite);

Here $mountingSite will be null and it is correct if indexing is disabled for the mount point, but setSite` expects a valis site.

To Reproduce
Steps to reproduce the behavior:

1. Create a new root page, which is a mount point to the existing root page.
2. Disable indexing via solr_enabled_read: false in the site config for that new site
3. Edit any page in the site -> exception

Used versions (please complete the following information):

• TYPO3 Version: 12.4.28
• EXT:solr Version: 12.0.1