[BUGFIX] Prevent type error on static route

If you happen to have inconsistent data in the
config.yaml like:

```
routes:
  -
    route: robots.txt
    type: staticText
    source: 'EXT:my_sitepackage_mysite/Resources/Public/robots.txt'
```

Currently a PHP type error is thrown instead of a proper exception.

Resolves: #102773
Releases: main, 12.4
Change-Id: If16a93b851237c9d39893cd3e56fa9693986721d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/82478


Tested-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>

typo3/sysext/frontend/Classes/Middleware/StaticRouteResolver.php

@@ -139,6 +139,9 @@ class StaticRouteResolver implements MiddlewareInterface
     {
         switch ($type) {
             case 'staticText':
+                if (!isset($routeConfig['content']) || !is_string($routeConfig['content'])) {
+                    throw new \InvalidArgumentException('A static route of type "staticText" must have a content defined.', 1704704705);
+                }
                 $content = $routeConfig['content'];
                 $contentType = 'text/plain; charset=utf-8';
                 break;