Fix issue with IndexError being raised by the StreamReader.iter_chunks() generator

[jlacoline]

What do these changes do?

Make StreamReader.readchunk method return b"" when buffer is empty eof is reached

Are there changes in behavior for the user?

Fixes the IndexError raised when using iter_chunks()

Related issue number

#2112

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
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    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
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    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #2113 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2113      +/-   ##
==========================================
+ Coverage   97.08%   97.11%   +0.02%     
==========================================
  Files          38       38              
  Lines        7692     7694       +2     
  Branches     1345     1346       +1     
==========================================
+ Hits         7468     7472       +4     
+ Misses        102      101       -1     
+ Partials      122      121       -1

Impacted Files	Coverage Δ
aiohttp/streams.py	98.75% <100%> (+0.5%)	⬆️

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[fafhrd91]

Thanks!

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