Remove unnecessary generic type arguments

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175155365

java/google/registry/flows/domain/DomainCheckFlow.java

@@ -187,7 +187,7 @@ public final class DomainCheckFlow implements Flow {
     }
 
     return reservationTypes.isEmpty()
-        ? Optional.<String>empty()
+        ? Optional.empty()
         : Optional.of(getTypeOfHighestSeverity(reservationTypes).getMessageForCheck());
   }