fix(TemplateManager): Make sure TemplateFolder is a Folder

[solracsf]

• Resolves: https://github.com/nextcloud/server/issues/49436

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [x] Backports requested where applicable (ex: critical bugfixes)

[joshtrichards]

LOL I just wrote basically the same code and was about to shoot over the PR:

    private function getTemplateFolder(): Node {
        if ($this->getTemplatePath() !== '') {
            $userTemplateFolder = $this->rootFolder->getUserFolder($this->userId)->get($this->getTemplatePath());
            if ($userTemplateFolder->getType() === \OCP\Files\FileInfo::TYPE_FOLDER) {
                return $userTemplateFolder;
            }
        }
        throw new NotFoundException();
    }

Both re-trigger the new template folder setup setting (good). I didn't make the exception message more verbose though. Since it's caught silently when used in getTemplateFiles() I think re-triggering the Create new template folder option is enough (but won't hurt!).