MINOR: prevent NullPointerException of polling two metrics in Kafka C…

…onnect

`assignmentSnapshot` may not always get initialized in some cases, especially when
Kafka connect is started from scratch. If `assignmentSnapshot` is not initialized,
blindly resgistering `assigned-connectors` and `assigned-tasks` metrics will cause
NullPointerException when the two metrics are polled.

the proposed fix is to return 0 in both measure() method if `assignmentSnapshot` is null

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();
                 }
             };