Fix Category#mutable default method to return copy

[knutwannheden]

The default implementation Category#mutable didn't actually return a mutable copy but a new "empty" mutable Category object.

[knutwannheden]

@aloubyansky Since we were still having the problem with null category IDs I took a look and I think I found the culprit.

[quarkus-bot]

Failing Jobs - Building 5a94f0e

Status	Name	Step	Failures	Logs	Raw logs
✖	Gradle Tests - JDK 11	Build	Failures	Logs	Raw logs
✔️	Gradle Tests - JDK 11 Windows

Full information is available in the Build summary check run.

Failures

⚙️ Gradle Tests - JDK 11 #

- Failing: integration-tests/gradle 

📦 integration-tests/gradle

✖ io.quarkus.gradle.devmode.CompositeBuildWithDependenciesDevModeTest.main line 24 - More details - Source on GitHub

org.awaitility.core.ConditionTimeoutException: Condition with lambda expression in io.quarkus.test.devmode.util.DevModeTestUtils that uses java.util.function.Supplier, java.util.function.Supplierjava.util.concurrent.atomic.AtomicReference, java.util.concurrent.atomic.AtomicReferencejava.lang.String, java.lang.Stringboolean was not fulfilled within 1 minutes.
	at org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
	at org.awaitility.core.CallableCondition.await(CallableCondition.java:78)

[aloubyansky]

Thanks @knutwannheden!