Fix NullPointerException in MockUncasedHostProvider (#28424)

The MockUncasedHostProvider accesses nodes that are not fully built yet, where TransportService.getNode() returns null, which means that the null entries end up in the list of seedNodes that UnicastZenPing then uses.

test/framework/src/main/java/org/elasticsearch/test/discovery/MockUncasedHostProvider.java

@@ -20,6 +20,7 @@ package org.elasticsearch.test.discovery;
 
 import org.elasticsearch.cluster.ClusterName;
 import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
 import org.elasticsearch.discovery.zen.UnicastHostsProvider;
 
@@ -27,6 +28,7 @@ import java.io.Closeable;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.Set;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
@@ -54,15 +56,19 @@ public final class MockUncasedHostProvider implements UnicastHostsProvider, Clos
 
     @Override
     public List<DiscoveryNode> buildDynamicNodes() {
+        final DiscoveryNode localNode = getNode();
+        assert localNode != null;
         synchronized (activeNodesPerCluster) {
             Set<MockUncasedHostProvider> activeNodes = getActiveNodesForCurrentCluster();
             return activeNodes.stream()
                 .map(MockUncasedHostProvider::getNode)
-                .filter(n -> !localNodeSupplier.get().equals(n))
+                .filter(Objects::nonNull)
+                .filter(n -> localNode.equals(n) == false)
                 .collect(Collectors.toList());
         }
     }
 
+    @Nullable
     private DiscoveryNode getNode() {
         return localNodeSupplier.get();
     }