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.

-------------
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);

java/google/registry/model/billing/BillingEvent.java

@@ -19,9 +19,11 @@ import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Preconditions.checkState;
 import static google.registry.model.ofy.ObjectifyService.ofy;
+import static google.registry.util.CollectionUtils.forceEmptyToNull;
 import static google.registry.util.CollectionUtils.nullToEmptyImmutableCopy;
 import static google.registry.util.CollectionUtils.union;
 import static google.registry.util.DateTimeUtils.END_OF_TIME;
+import static google.registry.util.PreconditionsUtils.checkArgumentNotNull;
 
 import com.google.common.base.Optional;
 import com.google.common.collect.ImmutableMap;
@@ -44,6 +46,7 @@ import google.registry.model.reporting.HistoryEntry;
 import google.registry.model.transfer.TransferData.TransferServerApproveEntity;
 import java.util.Objects;
 import java.util.Set;
+import javax.annotation.Nullable;
 import org.joda.money.Money;
 import org.joda.time.DateTime;
 
@@ -98,6 +101,7 @@ public abstract class BillingEvent extends ImmutableObject
   /** The fully qualified domain name of the domain that the bill is for. */
   String targetId;
 
+  @Nullable
   Set<Flag> flags;
 
   public String getClientId() {
@@ -168,7 +172,7 @@ public abstract class BillingEvent extends ImmutableObject
     }
 
     public B setFlags(ImmutableSet<Flag> flags) {
-      getInstance().flags = flags;
+      getInstance().flags = forceEmptyToNull(checkArgumentNotNull(flags, "flags"));
       return thisCastToDerived();
     }
 

javatests/google/registry/flows/domain/DomainCreateFlowTest.java

@@ -178,7 +178,7 @@ public class DomainCreateFlowTest extends ResourceFlowTestCase<DomainCreateFlow,
         : clock.nowUtc().plus(Registry.get(domainTld).getAddGracePeriodLength());
     ImmutableSet<BillingEvent.Flag> billingFlags = isAnchorTenant
         ? ImmutableSet.of(BillingEvent.Flag.ANCHOR_TENANT)
-        : null;
+        : ImmutableSet.<BillingEvent.Flag>of();
     HistoryEntry historyEntry = getHistoryEntries(domain).get(0);
     assertAboutDomains().that(domain)
         .hasRegistrationExpirationTime(