PubSubInboundChannelAdapter does not allow null payload

Hello, I would like to ask, if there is any possibility to receive message with empty payload via PubSubInboundChannelAdapter? It seems to me that this is impossible, however that is perfectly valid scenario in gcp pubsub.

```
java.lang.IllegalArgumentException: payload must not be null
at org.springframework.util.Assert.notNull(Assert.java:201)
at org.springframework.integration.support.MessageBuilder.(MessageBuilder.java:70)
at org.springframework.integration.support.MessageBuilder.withPayload(MessageBuilder.java:114)
at org.springframework.integration.support.DefaultMessageBuilderFactory.withPayload(DefaultMessageBuilderFactory.java:72)
at org.springframework.integration.support.DefaultMessageBuilderFactory.withPayload(DefaultMessageBuilderFactory.java:31)
at com.google.cloud.spring.pubsub.integration.inbound.PubSubInboundChannelAdapter.consumeMessage(PubSubInboundChannelAdapter.java:146)
at com.google.cloud.spring.pubsub.core.subscriber.PubSubSubscriberTemplate.lambda$subscribeAndConvert$1(PubSubSubscriberTemplate.java:173)
at com.google.cloud.pubsub.v1.MessageDispatcher$4.run(MessageDispatcher.java:396)
at com.google.cloud.pubsub.v1.SequentialExecutorService$AutoExecutor$1.run(SequentialExecutorService.java:134)
at com.google.cloud.pubsub.v1.SequentialExecutorService$SequentialExecutor$1.run(SequentialExecutorService.java:112)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
at java.base/java.lang.Thread.run(Thread.java:833)
```