Enforce nullness consistency on EppResponse.set...() methods

The callsites were inconsistent between whether they were passing empty list or
null, and many of the ones that were passing null were not correctly annotated
with @Nullable. I'm now going with empty list throughout except for the final
step where the actual field that will be transformed into XML is set, where it
is coerced to null to avoid an empty element in the XML output.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139340837

java/google/registry/model/eppoutput/EppResponse.java

@@ -14,6 +14,9 @@
 
 package google.registry.model.eppoutput;
 
+import static google.registry.util.CollectionUtils.forceEmptyToNull;
+import static google.registry.util.CollectionUtils.nullToEmptyImmutableCopy;
+
 import com.google.common.collect.FluentIterable;
 import com.google.common.collect.ImmutableList;
 import google.registry.model.Buildable;
@@ -150,11 +153,11 @@ public class EppResponse extends ImmutableObject implements ResponseOrGreeting {
   ImmutableList<? extends ResponseExtension> extensions;
 
   public ImmutableList<? extends ResponseData> getResponseData() {
-    return resData;
+    return nullToEmptyImmutableCopy(resData);
   }
 
   public ImmutableList<? extends ResponseExtension> getExtensions() {
-    return extensions;
+    return nullToEmptyImmutableCopy(extensions);
   }
 
   @Nullable
@@ -216,8 +219,8 @@ public class EppResponse extends ImmutableObject implements ResponseOrGreeting {
       return setMultipleResData(ImmutableList.of(onlyResData));
     }
 
-    public Builder setMultipleResData(@Nullable ImmutableList<? extends ResponseData> resData) {
-      getInstance().resData = resData;
+    public Builder setMultipleResData(ImmutableList<? extends ResponseData> resData) {
+      getInstance().resData = forceEmptyToNull(resData);
       return this;
     }
 
@@ -225,8 +228,8 @@ public class EppResponse extends ImmutableObject implements ResponseOrGreeting {
       return setExtensions(ImmutableList.of(onlyExtension));
     }
 
-    public Builder setExtensions(@Nullable ImmutableList<? extends ResponseExtension> extensions) {
-      getInstance().extensions = extensions;
+    public Builder setExtensions(ImmutableList<? extends ResponseExtension> extensions) {
+      getInstance().extensions = forceEmptyToNull(extensions);
       return this;
     }
   }