[fix][client] Fix print error log 'Auto getting partitions failed' wh…

…en expend partition. (apache#21485)

pulsar-client/src/main/java/org/apache/pulsar/client/impl/PartitionedProducerImpl.java

@@ -436,7 +436,7 @@ public CompletableFuture<Void> onTopicsExtended(Collection<String> topicsExtende
                                 });
                         // call interceptor with the metadata change
                         onPartitionsChange(topic, currentPartitionNumber);
-                        return null;
+                        return future;
                     }
                 } else {
                     log.error("[{}] not support shrink topic partitions. old: {}, new: {}",