test(pubsub): fix mock stubbing to prevent NPEs in StreamingSubscriberConnectionTest

Stub mock callables to return pending SettableApiFuture and default ClientStream
to avoid NullPointerException during connection startup and operation in tests.

TAG=agy
CONV=de3ef09d-a26c-4190-b578-4c993f807bdc

Author: lqiu96

java-pubsub/google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/StreamingSubscriberConnectionTest.java

@@ -104,7 +104,7 @@ public class StreamingSubscriberConnectionTest {
   private static final Duration MAX_ACK_EXTENSION_PERIOD = Duration.ofMinutes(60);
 
   @Before
-  public void setUp() {
+  public void setUp() throws Exception {
     systemExecutor = new FakeScheduledExecutorService();
     executor = new FakeScheduledExecutorService();
     clock = systemExecutor.getClock();
@@ -117,6 +117,15 @@ public void setUp() {
     when(mockSubscriberStub.streamingPullCallable()).thenReturn(mockStreamingCallable);
     when(mockSubscriberStub.acknowledgeCallable()).thenReturn(mockAcknowledgeCallable);
     when(mockSubscriberStub.modifyAckDeadlineCallable()).thenReturn(mockModifyAckDeadlineCallable);
+
+    ClientStream<StreamingPullRequest> defaultMockClientStream =
+        mock(ClientStream.class, withSettings().withoutAnnotations());
+    when(mockStreamingCallable.splitCall(any(), any())).thenReturn(defaultMockClientStream);
+
+    when(mockAcknowledgeCallable.futureCall(any()))
+        .thenAnswer(invocation -> SettableApiFuture.create());
+    when(mockModifyAckDeadlineCallable.futureCall(any()))
+        .thenAnswer(invocation -> SettableApiFuture.create());
   }
 
   @After