Return empty bytes with end-of-chunk marker in empty stream reader

[asvetlov]

Fixes #3186

[codecov-io]

Codecov Report

Merging #3295 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3295   +/-   ##
=======================================
  Coverage   98.03%   98.03%           
=======================================
  Files          43       43           
  Lines        8001     8001           
  Branches     1354     1354           
=======================================
  Hits         7844     7844           
  Misses         65       65           
  Partials       92       92

Impacted Files	Coverage Δ
aiohttp/streams.py	99.2% <100%> (ø)	⬆️

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[new issue]: https://github.com/aio-libs/aiohttp/issues/new