KAFKA-9219: prevent NullPointerException in Kafka Connect metrics (#7652

)

`assignmentSnapshot` may take a moment to get initialized in some cases, especially when
Kafka Connect is started from scratch. While `assignmentSnapshot` is not initialized, return 0 in both `measure()` methods.

Reviewers: Mickael Maison <mickael.maison@gmail.com>, Edoardo Comar <ecomar@uk.ibm.com>

connect/runtime/src/main/java/org/apache/kafka/connect/runtime/distributed/WorkerCoordinator.java

@@ -306,13 +306,19 @@ public WorkerCoordinatorMetrics(Metrics metrics, String metricGrpPrefix) {
             Measurable numConnectors = new Measurable() {
                 @Override
                 public double measure(MetricConfig config, long now) {
+                    if (assignmentSnapshot == null) {
+                        return 0.0;
+                    }
                     return assignmentSnapshot.connectors().size();
                 }
             };
 
             Measurable numTasks = new Measurable() {
                 @Override
                 public double measure(MetricConfig config, long now) {
+                    if (assignmentSnapshot == null) {
+                        return 0.0;
+                    }
                     return assignmentSnapshot.tasks().size();
                 }
             };