MINOR: Optimize map lookup efficiency with getOrDefault (#20229)

Optimized `getRemainingRecords()` method by replacing inefficient
`containsKey() + get()` pattern with `getOrDefault()` to reduce map
lookups from 2 to 1 per partition.

Reviewers: Chia-Ping Tsai <[email protected]>

Author: Kguswo

examples/src/main/java/kafka/examples/ExactlyOnceMessageProcessor.java

@@ -216,11 +216,7 @@ private long getRemainingRecords(KafkaConsumer<Integer, String> consumer) {
         }
         return consumer.assignment().stream().mapToLong(partition -> {
             long currentPosition = consumer.position(partition);
-            if (fullEndOffsets.containsKey(partition)) {
-                return fullEndOffsets.get(partition) - currentPosition;
-            } else {
-                return 0;
-            }
+            return fullEndOffsets.getOrDefault(partition, currentPosition) - currentPosition;
         }).sum();
     }