Fix Java 7 missing generic type error

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154081069

java/google/registry/flows/FlowReporter.java

@@ -105,7 +105,7 @@ public class FlowReporter {
   private static final Optional<String> extractTld(String domainName) {
     int index = domainName.indexOf('.');
     return index == -1
-        ? Optional.absent()
+        ? Optional.<String>absent()
         : Optional.of(Ascii.toLowerCase(domainName.substring(index + 1)));
   }