Improve pretty-print diffing of Datastore entities

This removes the countless lines of the form "[null, []]" in registry_tool diffs
that are an artifact of the way we handle nulls in Objectify.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133409440

javatests/google/registry/xml/XmlTestUtils.java

@@ -16,7 +16,7 @@ package google.registry.xml;
 
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.truth.Truth.assert_;
-import static google.registry.util.DiffUtils.prettyPrintDeepDiff;
+import static google.registry.util.DiffUtils.prettyPrintXmlDeepDiff;
 import static org.joda.time.DateTimeZone.UTC;
 
 import com.google.common.base.Splitter;
@@ -59,7 +59,7 @@ public class XmlTestUtils {
           message,
           expected,
           actual,
-          prettyPrintDeepDiff(expectedMap, actualMap, null)));
+          prettyPrintXmlDeepDiff(expectedMap, actualMap, null)));
     }
   }