Don't allow null in BillingEvent.setFlags()

It's better that it always takes a non-null ImmutableSet, which may either be
empty or contain elements.  That way the ugliness of nullness is contained
just to the entity class itself, and all other code that interacts with it
can always be assured of having a real set to deal with.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=132066238

java/google/registry/flows/domain/DomainCreateFlow.java

@@ -157,7 +157,9 @@ public class DomainCreateFlow extends DomainCreateOrAllocateFlow {
         .setBillingTime(now.plus(isAnchorTenant()
             ? registry.getAnchorTenantAddGracePeriodLength()
             : registry.getAddGracePeriodLength()))
-        .setFlags(isAnchorTenant() ? ImmutableSet.of(BillingEvent.Flag.ANCHOR_TENANT) : null)
+        .setFlags(isAnchorTenant()
+            ? ImmutableSet.of(BillingEvent.Flag.ANCHOR_TENANT)
+            : ImmutableSet.<BillingEvent.Flag>of())
         .setParent(historyEntry)
         .build();
     ofy().save().entity(createEvent);