MINOR: Clean up unreachable code in FetcherTest (#19376)

This is from [#16532's comment](https://github.com/apache/kafka/pull/16532/files#r2028985028):

The forEach loop in the assertion will never execute because
`nonResponseData` is empty.

This happens because the above assertion `emptyMap()` has a size of 0,
so there are no elements to iterate over.

Reviewers: PoAn Yang <[email protected]>, Ken Huang
<[email protected]>, TaiJuWu <[email protected]>, TengYao Chi
<[email protected]>, Chia-Ping Tsai <[email protected]>

Author: Parkerhiphop

clients/src/test/java/org/apache/kafka/clients/consumer/internals/FetcherTest.java

@@ -3662,9 +3662,8 @@ public void testFetcherDontCacheAnyData() {
         LinkedHashMap<TopicPartition, FetchResponseData.PartitionData> responseData = fetchResponse.responseData(topicNames, version);
         assertEquals(topicNames.size(), responseData.size());
         responseData.forEach((topicPartition, partitionData) -> assertEquals(records, partitionData.records()));
-        LinkedHashMap<TopicPartition, FetchResponseData.PartitionData> nonResponseData = fetchResponse.responseData(emptyMap(), version);
-        assertEquals(emptyMap().size(), nonResponseData.size());
-        nonResponseData.forEach((topicPartition, partitionData) -> assertEquals(MemoryRecords.EMPTY, partitionData.records()));
+        LinkedHashMap<TopicPartition, FetchResponseData.PartitionData> nonResponseData = fetchResponse.responseData(Map.of(), version);
+        assertTrue(nonResponseData.isEmpty());
     }
 
     private OffsetsForLeaderEpochResponse prepareOffsetsForLeaderEpochResponse(