Qualify the usage of ImmutableMap.of()

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167723174

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

@@ -300,9 +300,7 @@ public class DomainTransferApproveFlowTest
                     .build())
             .toArray(BillingEvent.class));
     // There should be no grace period.
-    assertGracePeriods(
-        domain.getGracePeriods(),
-        ImmutableMap.of());
+    assertGracePeriods(domain.getGracePeriods(), ImmutableMap.<GracePeriod, BillingEvent>of());
   }
 
   private void doSuccessfulTest(String tld, String commandFilename, String expectedXmlFilename)