assert from write_ready

[billpriest]

I have a low bandwidth serial read/write (read ~16 bytes at 0.5 Hz/~16 bytes at 1.0 Hz). I've confirmed that I only have one transport.write and that the amount of data being written is always > 0. Periodically, I see a stack trace where an is raised
assert data, 'Write buffer should not be empty'
I added if len(data) == 0: print("len = 0") and set a breakpoint on it. The callstack showed the following
_run, events.py:80
_run_once, base_events.py:1890
run_forever, base_events.py:596

I set breakpoints on connection lost and didn't hit that and I don't believe that the transport was closed.