[Test] Make unable master node error more explicit

Instead of NPE, this PR throws an AssertionError with more explicit
error message.

Relates: elastic#127213

Author: ywangd

test/framework/src/main/java/org/elasticsearch/test/InternalTestCluster.java

@@ -2040,7 +2040,11 @@ public String getMasterName(@Nullable String viaNode) {
         try {
             ClusterServiceUtils.awaitClusterState(logger, state -> state.nodes().getMasterNode() != null, clusterService(viaNode));
             final ClusterState state = client(viaNode).admin().cluster().prepareState(TEST_REQUEST_TIMEOUT).setLocal(true).get().getState();
-            return state.nodes().getMasterNode().getName();
+            final DiscoveryNode masterNode = state.nodes().getMasterNode();
+            if (masterNode == null) {
+                throw new AssertionError("Master is not stable but the method expects a stable master node");
+            }
+            return masterNode.getName();
         } catch (Exception e) {
             logger.warn("Can't fetch cluster state", e);
             throw new RuntimeException("Can't get master node " + e.getMessage(), e);