KAFKA-9984 Should fail the subscription when pattern is empty (#8665)

Reviewers: Boyang Chen <boyang@confluent.io>, Chia-Ping Tsai <chia7712@gmail.com>, Matthias J. Sax <matthias@confluent.io>

clients/src/main/java/org/apache/kafka/clients/consumer/KafkaConsumer.java

@@ -1015,8 +1015,9 @@ public class KafkaConsumer<K, V> implements Consumer<K, V> {
     @Override
     public void subscribe(Pattern pattern, ConsumerRebalanceListener listener) {
         maybeThrowInvalidGroupIdException();
-        if (pattern == null)
-            throw new IllegalArgumentException("Topic pattern to subscribe to cannot be null");
+        if (pattern == null || pattern.toString().equals(""))
+            throw new IllegalArgumentException("Topic pattern to subscribe to cannot be " + (pattern == null ?
+                    "null" : "empty"));
 
         acquireAndEnsureOpen();
         try {

clients/src/test/java/org/apache/kafka/clients/consumer/KafkaConsumerTest.java

@@ -282,6 +282,13 @@ public class KafkaConsumerTest {
         }
     }
 
+    @Test(expected = IllegalArgumentException.class)
+    public void testSubscriptionOnEmptyPattern() {
+        try (KafkaConsumer<byte[], byte[]> consumer = newConsumer(groupId)) {
+            consumer.subscribe(Pattern.compile(""));
+        }
+    }
+
     @Test(expected = IllegalStateException.class)
     public void testSubscriptionWithEmptyPartitionAssignment() {
         Properties props = new Properties();