[BUG] Runtime error when OAI-PMH URL is called without verb (regression)

[stweil]

Description

The OAI-PMH URL should show an error code (badVerb) if it is used without any verb.
Working example: https://digi.bib.uni-mannheim.de/oai/.

With TYPO3 11 and release 5.0.7 it creates an oops runtime error.

Reproduction

Steps to reproduce the behaviour:

1. Configure page with OAI-PMH plugin
2. Load URL of newly created page in web browser without adding the required verb
3. See error

Expected Behavior

No runtime error.

[stweil]

If a verb is given, the plugin produces the expected OAI-PMH XML.

[stweil]

With a recent 5.1.x, I don't get a runtime error if the OAI-PMH URL is called without a verb and the TYPO3 caches were flushed, but the runtime error occurs for each following access to the same URL which now is served from cache.

The current main branch does not show this issue.