KAFKA-15315: Use getOrDefault rather than get in TransactionState (#14167)

Reviewers: Ziming Deng dengziming1993@gmail.com.

clients/src/main/java/org/apache/kafka/clients/admin/TransactionState.java

@@ -49,8 +49,7 @@ public enum TransactionState {
     }
 
     public static TransactionState parse(String name) {
-        TransactionState state = NAME_TO_ENUM.get(name);
-        return state == null ? UNKNOWN : state;
+        return NAME_TO_ENUM.getOrDefault(name, UNKNOWN);
     }
 
 }