[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/+/82367


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>