Avoid producing empty AdditionalIndexedClassesBuildItem

Even if it doesn't complain right now,
that's certainly something that could break later

extensions/mongodb-client/deployment/pom.xml

@@ -91,6 +91,11 @@
             <artifactId>awaitility</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>org.assertj</groupId>
             <artifactId>assertj-core</artifactId>

extensions/mongodb-client/deployment/src/main/java/io/quarkus/mongodb/deployment/MongoClientProcessor.java

@@ -126,18 +126,18 @@ AdditionalIndexedClassesBuildItem includeMongoCommandListener(MongoClientBuildTi
     }
 
     @BuildStep
-    AdditionalIndexedClassesBuildItem includeMongoCommandMetricListener(
+    void includeMongoCommandMetricListener(
+            BuildProducer<AdditionalIndexedClassesBuildItem> additionalIndexedClasses,
             MongoClientBuildTimeConfig buildTimeConfig,
             Optional<MetricsCapabilityBuildItem> metricsCapability) {
         if (!buildTimeConfig.metricsEnabled) {
-            return new AdditionalIndexedClassesBuildItem();
+            return;
         }
         boolean withMicrometer = metricsCapability.map(cap -> cap.metricsSupported(MetricsFactory.MICROMETER))
                 .orElse(false);
         if (withMicrometer) {
-            return new AdditionalIndexedClassesBuildItem(MicrometerCommandListener.class.getName());
+            additionalIndexedClasses.produce(new AdditionalIndexedClassesBuildItem(MicrometerCommandListener.class.getName()));
         }
-        return new AdditionalIndexedClassesBuildItem();
     }
 
     @BuildStep

extensions/mongodb-client/deployment/src/test/java/io/quarkus/mongodb/deployment/MongoClientProcessorTest.java

@@ -1,19 +1,24 @@
 package io.quarkus.mongodb.deployment;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.*;
 
 import java.util.Optional;
 
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
+import org.mockito.ArgumentCaptor;
 
+import io.quarkus.deployment.annotations.BuildProducer;
 import io.quarkus.deployment.builditem.AdditionalIndexedClassesBuildItem;
 import io.quarkus.deployment.metrics.MetricsCapabilityBuildItem;
 import io.quarkus.runtime.metrics.MetricsFactory;
 
 class MongoClientProcessorTest {
     private final MongoClientProcessor buildStep = new MongoClientProcessor();
 
+    @SuppressWarnings("unchecked")
     @ParameterizedTest
     @CsvSource({
             "true, true, true", // Metrics enabled and Micrometer supported
@@ -25,13 +30,16 @@ void testIncludeMongoCommandMetricListener(boolean metricsEnabled, boolean micro
         MongoClientBuildTimeConfig config = config(metricsEnabled);
         Optional<MetricsCapabilityBuildItem> capability = capability(metricsEnabled, micrometerSupported);
 
-        AdditionalIndexedClassesBuildItem result = buildStep.includeMongoCommandMetricListener(config, capability);
+        BuildProducer<AdditionalIndexedClassesBuildItem> buildProducer = mock(BuildProducer.class);
+        buildStep.includeMongoCommandMetricListener(buildProducer, config, capability);
 
         if (expectedResult) {
-            assertThat(result.getClassesToIndex())
+            var captor = ArgumentCaptor.forClass(AdditionalIndexedClassesBuildItem.class);
+            verify(buildProducer, times(1)).produce(captor.capture());
+            assertThat(captor.getAllValues().get(0).getClassesToIndex())
                     .containsExactly("io.quarkus.mongodb.metrics.MicrometerCommandListener");
         } else {
-            assertThat(result.getClassesToIndex()).isEmpty();
+            verify(buildProducer, never()).produce(any(AdditionalIndexedClassesBuildItem.class));
         }
     }