Allow a credit of 0 when deleting a domain during a grace period (#1984)

There can be situations (anchor tenants, test tokens, other ways of
getting a domain to cost $0) where we may want to delete a domain during
the add grace period but the credit applied is 0. We should not fail on
those cases.

See b/277115241 for an example.

core/src/main/java/google/registry/model/domain/fee/Credit.java

@@ -34,7 +34,7 @@ public class Credit extends BaseFee {
       BigDecimal cost, FeeType type, String description) {
     Credit instance = new Credit();
     instance.cost = checkNotNull(cost);
-    checkArgument(instance.cost.signum() < 0);
+    checkArgument(instance.cost.signum() <= 0, "A credit cannot have a positive cost");
     instance.type = checkNotNull(type);
     instance.description = description;
     return instance;