Handle nullness properly in some message/flow/poll code

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127543268

java/google/registry/flows/Flow.java

@@ -27,6 +27,7 @@ import google.registry.model.eppoutput.Result;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
+import javax.annotation.Nullable;
 import org.joda.time.DateTime;
 
 /**
@@ -78,8 +79,8 @@ public abstract class Flow {
 
   protected EppOutput createOutput(
       Result.Code code,
-      ResponseData responseData,
-      ImmutableList<? extends ResponseExtension> extensions) {
+      @Nullable ResponseData responseData,
+      @Nullable ImmutableList<? extends ResponseExtension> extensions) {
     return EppOutput.create(new EppResponse.Builder()
         .setTrid(trid)
         .setResult(Result.create(code))

java/google/registry/flows/ResourceDeleteFlow.java

@@ -25,6 +25,7 @@ import google.registry.model.eppoutput.EppOutput;
 import google.registry.model.eppoutput.EppResponse.ResponseExtension;
 import google.registry.model.eppoutput.Result.Code;
 import java.util.Set;
+import javax.annotation.Nullable;
 
 /**
  * An EPP flow that deletes an {@link EppResource}.
@@ -58,6 +59,7 @@ public abstract class ResourceDeleteFlow<R extends EppResource, C extends Single
   }
 
   /** Subclasses can override this to return response extensions. */
+  @Nullable
   protected ImmutableList<? extends ResponseExtension> getDeleteResponseExtensions() {
     return null;
   }

java/google/registry/flows/poll/PollFlow.java

@@ -31,6 +31,7 @@ import google.registry.model.eppoutput.Result;
 import google.registry.model.poll.MessageQueueInfo;
 import google.registry.model.poll.PollMessage;
 import java.util.List;
+import javax.annotation.Nullable;
 
 /** Base class of EPP Poll command flows. Mostly provides datastore helper methods. */
 public abstract class PollFlow extends LoggedInFlow {
@@ -74,8 +75,8 @@ public abstract class PollFlow extends LoggedInFlow {
   protected EppOutput createOutput(
       Result.Code code,
       MessageQueueInfo messageQueueInfo,
-      ImmutableList<ResponseData> responseData,
-      ImmutableList<ResponseExtension> responseExtensions) {
+      @Nullable ImmutableList<ResponseData> responseData,
+      @Nullable ImmutableList<ResponseExtension> responseExtensions) {
     return EppOutput.create(new EppResponse.Builder()
         .setTrid(trid)
         .setResult(Result.create(code))