Fix NullPointerException in StaticPremiumPricingEngine (#1164)

* Fix NullPointerException in StaticPremiumPricingEngine

* Make getPremiumList return optional

* add isPresent checks

core/src/main/java/google/registry/export/ExportPremiumTermsAction.java

@@ -113,7 +113,7 @@ public class ExportPremiumTermsAction implements Runnable {
           "Skipping premium terms export for TLD %s because Drive folder isn't specified", tld);
       return Optional.of("Skipping export because no Drive folder is associated with this TLD");
     }
-    if (registry.getPremiumList() == null) {
+    if (!registry.getPremiumList().isPresent()) {
       logger.atInfo().log("No premium terms to export for TLD %s", tld);
       return Optional.of("No premium lists configured");
     }
@@ -137,7 +137,8 @@ public class ExportPremiumTermsAction implements Runnable {
   }
 
   private String getFormattedPremiumTerms(Registry registry) {
-    String premiumListName = registry.getPremiumList().getName();
+    checkState(registry.getPremiumList().isPresent(), "%s does not have a premium list", tld);
+    String premiumListName = registry.getPremiumList().get().getName();
     checkState(
         PremiumListDao.getLatestRevision(premiumListName).isPresent(),
         "Could not load premium list for " + tld);

core/src/main/java/google/registry/model/pricing/StaticPremiumListPricingEngine.java

@@ -38,8 +38,11 @@ public final class StaticPremiumListPricingEngine implements PremiumPricingEngin
     String tld = getTldFromDomainName(fullyQualifiedDomainName);
     String label = InternetDomainName.from(fullyQualifiedDomainName).parts().get(0);
     Registry registry = Registry.get(checkNotNull(tld, "tld"));
-    Optional<Money> premiumPrice =
-        PremiumListDao.getPremiumPrice(registry.getPremiumList().getName(), label);
+    Optional<Money> premiumPrice = Optional.empty();
+    if (registry.getPremiumList().isPresent()) {
+      premiumPrice =
+          PremiumListDao.getPremiumPrice(registry.getPremiumList().get().getName(), label);
+    }
     return DomainPrices.create(
         premiumPrice.isPresent(),
         premiumPrice.orElse(registry.getStandardCreateCost()),

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

@@ -607,8 +607,8 @@ public class Registry extends ImmutableObject implements Buildable, DatastoreAnd
   }
 
   @Nullable
-  public Key<PremiumList> getPremiumList() {
-    return premiumList;
+  public Optional<Key<PremiumList>> getPremiumList() {
+    return Optional.ofNullable(premiumList);
   }
 
   public CurrencyUnit getCurrency() {

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

@@ -263,7 +263,7 @@ public final class PremiumList extends BaseDomainLabelList<Money, PremiumList.Pr
 
   @Override
   public boolean refersToKey(Registry registry, Key<? extends BaseDomainLabelList<?, ?>> key) {
-    return Objects.equals(registry.getPremiumList(), key);
+    return Objects.equals(registry.getPremiumList().orElse(null), key);
   }
 
   @Override