Avoid null pointer exception when resolving generics in RR

[Sgitario]

This pull request does not provide support for generics when using sub resources, but avoid giving a null pointer exception. I found this issue when looking at #30127.

[quarkus-bot]

Failing Jobs - Building 94c8dd3

Status	Name	Step	Failures	Logs	Raw logs
✔️	JVM Tests - JDK 11
✔️	JVM Tests - JDK 17
✖	JVM Tests - JDK 17 Windows	Build	Failures	Logs	Raw logs
✔️	JVM Tests - JDK 19

Full information is available in the Build summary check run.

Failures

⚙️ JVM Tests - JDK 17 Windows #

- Failing: extensions/cache/deployment 
! Skipped: extensions/csrf-reactive/deployment extensions/qute/deployment extensions/redis-cache/deployment and 7 more

📦 extensions/cache/deployment

✖ io.quarkus.cache.test.runtime.ConcurrencyTest.testConcurrentCacheAccessWithLockTimeout line 67 - More details - Source on GitHub

org.opentest4j.AssertionFailedError: expected: not equal but was: <pool-48-thread-1>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:152)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)

[Sgitario]

I think the test failures are unrelated. Merging.