[PR #5393/c6eb5307 backport][3.8] Fix content-type on empty string.

[patchback]

This is a backport of PR #5393 as merged into master (c6eb530).

Fixes #5392

[codecov]

Codecov Report

Merging #6121 (c6eb530) into 3.8 (66e281f) will decrease coverage by 4.19%.
The diff coverage is 90.32%.

❗ Current head c6eb530 differs from pull request most recent head f62cf3b. Consider uploading reports for the commit f62cf3b to get more accurate results

@@            Coverage Diff             @@
##              3.8    #6121      +/-   ##
==========================================
- Coverage   97.52%   93.33%   -4.20%     
==========================================
  Files          44      102      +58     
  Lines        8865    30080   +21215     
  Branches     1429     2690    +1261     
==========================================
+ Hits         8646    28076   +19430     
- Misses        103     1829    +1726     
- Partials      116      175      +59     

Flag	Coverage Δ
unit	93.26% <90.13%> (-4.09%)	⬇️

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

Impacted Files	Coverage Δ
aiohttp/web_fileresponse.py	98.62% <ø> (-1.38%)	⬇️
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	95.99% <ø> (-1.57%)	⬇️
aiohttp/web_response.py	98.21% <ø> (-0.06%)	⬇️
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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