Don't fail if an error response is missing during publishing

[NathanHowell]

HttpURLConnection.getErrorStream is allowed to return null, and when this happens an NPE is raised:

INFO: Uploading to https://...
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NullPointerException
	at java.base/java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:395)
	at java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:2022)
	at rules.jvm.external.maven.MavenPublisher.main(MavenPublisher.java:99)
Caused by: java.lang.NullPointerException
	at rules.jvm.external.ByteStreams.copy(ByteStreams.java:26)
	at rules.jvm.external.ByteStreams.toByteArray(ByteStreams.java:18)
	at rules.jvm.external.maven.MavenPublisher.lambda$httpUpload$1(MavenPublisher.java:219)
	at rules.jvm.external.maven.MavenPublisher.lambda$upload$0(MavenPublisher.java:182)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

[jin]

thanks!