Add more nullability annotations to module/spring-boot-amqp

See gh-46587

Author: mhalbritter

documentation/spring-boot-docs/src/main/kotlin/org/springframework/boot/docs/actuator/endpoints/health/reactivehealthindicators/MyReactiveHealthIndicator.kt

@@ -26,15 +26,15 @@ class MyReactiveHealthIndicator : ReactiveHealthIndicator {
 
 	override fun health(): Mono<Health> {
 		// @formatter:off
-		return doHealthCheck()!!.onErrorResume { exception: Throwable? ->
+		return doHealthCheck().onErrorResume { exception: Throwable ->
 			Mono.just(Health.Builder().down(exception).build())
 		}
 		// @formatter:on
 	}
 
-	private fun doHealthCheck(): Mono<Health>? {
+	private fun doHealthCheck(): Mono<Health> {
 		// perform some specific health check
-		return  /**/ null
+		return  /**/ Mono.empty()
 	}
 
 }

module/spring-boot-amqp/src/main/java/org/springframework/boot/amqp/health/RabbitHealthIndicator.java

@@ -43,7 +43,11 @@ public RabbitHealthIndicator(RabbitTemplate rabbitTemplate) {
 
 	@Override
 	protected void doHealthCheck(Health.Builder builder) throws Exception {
-		builder.up().withDetail("version", getVersion());
+		builder.up();
+		String version = getVersion();
+		if (version != null) {
+			builder.withDetail("version", version);
+		}
 	}
 
 	private @Nullable String getVersion() {