[5.x] Stop remote blob upload if upload is complete.

[benjaminp]

If a ByteStream/Write RPC fails, but ByteStream/QueryWriteStatus reveals the upload is in fact complete, avoid a NullPointerException. This CL is the dual fix of 78b89a0 for uploads.

On bazel-6.0.0-pre.20211117.1, I observed:

java.lang.NullPointerException
        at com.google.devtools.build.lib.remote.Chunker.seek(Chunker.java:156)
        at com.google.devtools.build.lib.remote.ByteStreamUploader$AsyncUpload.lambda$start$0(ByteStreamUploader.java:416)
        at com.google.devtools.build.lib.remote.Retrier.executeAsync(Retrier.java:277)
        at com.google.devtools.build.lib.remote.Retrier.lambda$onExecuteAsyncFailure$1(Retrier.java:293)
        at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleAsyncTask.runInterruptibly(TrustedListenableFutureTask.java:160)
        at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleAsyncTask.runInterruptibly(TrustedListenableFutureTask.java:143)
        at com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:69)
        at com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:78)
        at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleTask.runInterruptibly(TrustedListenableFutureTask.java:125)
        at com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:69)
        at com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:78)
        at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
        at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(Unknown Source)

Cherry-pick of 72d5255.