s3: Avoid pwrite calls for the 1-byte writes with zero content

s3: Avoid pwrite calls for the 1-byte writes with zero content

When a write cache is empty and we write one 0, there is no point in actually
writing into the write cache, which would trigger a potentially superfluous
write cache flush. We can rely on posix semantics to give us 0's when
subsequent read calls want to read from file areas not written yet.

Jeremy, please check!