Small SQL persistence fixes to model classes (#863)

* Small SQL persistence fixes to model classes

- Add a createVKey() method to Registry (Registry vkeys are composite)
- Add/fix toSqlEntities() methods in premium and reserved list classes.

* Remove fixes addressed by #866

core/src/main/java/google/registry/model/registry/Registry.java

@@ -285,6 +285,10 @@ public class Registry extends ImmutableObject implements Buildable, DatastoreAnd
     return VKey.create(Registry.class, tld, Key.create(getCrossTldKey(), Registry.class, tld));
   }
 
+  public static VKey<Registry> createVKey(Key<Registry> key) {
+    return createVKey(key.getName());
+  }
+
   /**
    * The name of the pricing engine that this TLD uses.
    *

core/src/main/java/google/registry/model/registry/label/PremiumList.java

@@ -352,7 +352,7 @@ public final class PremiumList extends BaseDomainLabelList<Money, PremiumList.Pr
 
     @Override
     public ImmutableList<SqlEntity> toSqlEntities() {
-      return null;
+      return ImmutableList.of();
     }
 
     /** A builder for constructing {@link PremiumListEntry} objects, since they are immutable. */