Use an empty interval instead of null to signal no LRP

This removes the need for some null checks

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=136146549

java/google/registry/model/registry/Registry.java

@@ -574,7 +574,7 @@ public class Registry extends ImmutableObject implements Buildable {
   @Nullable
   public Interval getLrpPeriod() {
     return (lrpPeriodStart == null && lrpPeriodEnd == null)
-        ? null
+        ? new Interval(START_OF_TIME, Duration.ZERO)
         : new Interval(lrpPeriodStart, lrpPeriodEnd);
   }