[FLINK-26596][runtime][test] Adds leadership loss handling

If the ZK connection is flaky, we might collect another
ZK connection loss. This is now handled properly.

Author: XComp

flink-runtime/src/test/java/org/apache/flink/runtime/leaderretrieval/ZooKeeperLeaderRetrievalConnectionHandlingTest.java

@@ -287,9 +287,10 @@ public void testNewLeaderAfterReconnectTriggersListenerNotification() throws Exc
                     // check that we find the new leader information eventually
                     CommonTestUtils.waitUntilCondition(
                             () -> {
-                                final CompletableFuture<String> afterConnectionReconnect =
-                                        queueLeaderElectionListener.next();
-                                return afterConnectionReconnect.get().equals(newLeaderAddress);
+                                final String afterConnectionReconnect =
+                                        queueLeaderElectionListener.next().get();
+                                return afterConnectionReconnect != null
+                                        && afterConnectionReconnect.equals(newLeaderAddress);
                             },
                             Deadline.fromNow(Duration.ofSeconds(30L)));
                 });