[PR #5380/11b46df4 backport][3.8] Fix sending zero byte files after sendfile changes

[patchback]

This is a backport of PR #5380 as merged into master (11b46df).

What do these changes do?

Changing the sendfile implementation in #5157 caused a
regression with sending zero byte files, and the test
had too much mocking to expose the issue.

Are there changes in behavior for the user?

Bugfix only

Related issue number

#5157
#5124
home-assistant/core#42514

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
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .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]

Codecov Report

Merging #6145 (11b46df) into 3.8 (66e281f) will decrease coverage by 4.20%.
The diff coverage is 90.45%.

❗ Current head 11b46df differs from pull request most recent head 0ed235d. Consider uploading reports for the commit 0ed235d to get more accurate results

@@            Coverage Diff             @@
##              3.8    #6145      +/-   ##
==========================================
- Coverage   97.52%   93.32%   -4.21%     
==========================================
  Files          44      102      +58     
  Lines        8865    30166   +21301     
  Branches     1429     2699    +1270     
==========================================
+ Hits         8646    28151   +19505     
- Misses        103     1840    +1737     
- Partials      116      175      +59     

Flag	Coverage Δ
unit	93.25% <90.26%> (-4.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	97.93% <ø> (-2.07%)	⬇️
aiohttp/web_log.py	100.00% <ø> (ø)
aiohttp/web_middlewares.py	100.00% <ø> (ø)
aiohttp/web_protocol.py	86.41% <ø> (-4.20%)	⬇️
aiohttp/web_request.py	95.91% <ø> (-1.65%)	⬇️
aiohttp/web_response.py	98.41% <ø> (+0.14%)	⬆️
aiohttp/web_routedef.py	98.11% <ø> (+0.01%)	⬆️
aiohttp/web_runner.py	92.34% <ø> (-5.29%)	⬇️
aiohttp/web_server.py	94.28% <ø> (-5.72%)	⬇️
aiohttp/web_urldispatcher.py	97.54% <ø> (-0.06%)	⬇️
... and 135 more

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