Don't create fallback policy if default target is null.

Author: kannanjgithub

rls/src/main/java/io/grpc/rls/CachingRlsLbClient.java

@@ -229,8 +229,12 @@ private CachingRlsLbClient(Builder builder) {
             new BackoffRefreshListener());
     // TODO(creamsoup) wait until lb is ready
     String defaultTarget = lbPolicyConfig.getRouteLookupConfig().defaultTarget();
-    logger.log(ChannelLogLevel.DEBUG, "starting fallback to {0}", defaultTarget);
-    fallbackChildPolicyWrapper = refCountedChildPolicyWrapperFactory.createOrGet(defaultTarget);
+    if (defaultTarget != null && !defaultTarget.isEmpty()) {
+      logger.log(ChannelLogLevel.DEBUG, "starting fallback to {0}", defaultTarget);
+      fallbackChildPolicyWrapper = refCountedChildPolicyWrapperFactory.createOrGet(defaultTarget);
+    } else {
+      fallbackChildPolicyWrapper = null;
+    }
 
     gaugeRegistration = helper.getMetricRecorder()
         .registerBatchCallback(new BatchCallback() {

rls/src/test/java/io/grpc/rls/RlsLoadBalancerTest.java

@@ -257,7 +257,7 @@ public void lb_working_withDefaultTarget_rlsResponding() throws Exception {
     inOrder.verifyNoMoreInteractions();
 
     assertThat(res.getStatus().isOk()).isTrue();
-    assertThat(subchannels).hasSize(1);
+    assertThat(subchannels).hasSize(2); // includes fallback sub-channel
     FakeSubchannel searchSubchannel = subchannels.getLast();
     assertThat(subchannelIsReady(searchSubchannel)).isFalse();
 
@@ -277,7 +277,7 @@ public void lb_working_withDefaultTarget_rlsResponding() throws Exception {
     // other rls picker itself is ready due to first channel.
     assertThat(res.getStatus().isOk()).isTrue();
     assertThat(subchannelIsReady(res.getSubchannel())).isFalse();
-    assertThat(subchannels).hasSize(2);
+    assertThat(subchannels).hasSize(3); // includes fallback sub-channel
     FakeSubchannel rescueSubchannel = subchannels.getLast();
 
     // search subchannel is down, rescue subchannel is connecting
@@ -545,7 +545,7 @@ public void lb_nameResolutionFailed() throws Exception {
     assertThat(subchannelIsReady(res.getSubchannel())).isFalse();
 
     inOrder.verify(helper).createSubchannel(any(CreateSubchannelArgs.class));
-    assertThat(subchannels).hasSize(1);
+    assertThat(subchannels).hasSize(1); // includes fallback sub-channel
 
     FakeSubchannel searchSubchannel = subchannels.getLast();
     searchSubchannel.updateState(ConnectivityStateInfo.forNonError(ConnectivityState.READY));