Prevent NPE in setAllHeaders

[Sgitario]

I didn't add any test to cover this change because it's easier to fix than to replicate the conditions to reproduce it in either the extension or an integration test.
Still, let me know if you want it to cover it and maybe I can add wiremock in the integration-tests/rest-client-reactive.
Fix #28922

[geoand]

I think we can live without a test for this change :)

[quarkus-bot]

Failing Jobs - Building 1d074d6

Status	Name	Step	Failures	Logs	Raw logs
✔️	Gradle Tests - JDK 11
✖	Gradle Tests - JDK 11 Windows	Build	Failures	Logs	Raw logs
✖	JVM Tests - JDK 11	Build	Failures	Logs	Raw logs
✔️	JVM Tests - JDK 17
✔️	JVM Tests - JDK 17 MacOS M1
✔️	JVM Tests - JDK 18

Full information is available in the Build summary check run.

Failures

⚙️ Gradle Tests - JDK 11 Windows #

- Failing: integration-tests/gradle 

📦 integration-tests/gradle

✖ io.quarkus.gradle.devmode.BasicJavaLibraryModuleDevModeTest.main line 23 - 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 was not fulfilled within 3 minutes.
	at org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
	at org.awaitility.core.CallableCondition.await(CallableCondition.java:78)

---

⚙️ JVM Tests - JDK 11 #

- Failing: integration-tests/opentelemetry-reactive-messaging 

📦 integration-tests/opentelemetry-reactive-messaging

✖ io.quarkus.it.opentelemetry.OpenTelemetryTestCase.testProcessorTracing line 95 - More details - Source on GitHub

org.awaitility.core.ConditionTimeoutException: Condition with lambda expression in io.quarkus.it.opentelemetry.OpenTelemetryTestCase was not fulfilled within 1 minutes.
	at org.awaitility.core.ConditionAwaiter.await(ConditionAwaiter.java:167)
	at org.awaitility.core.CallableCondition.await(CallableCondition.java:78)