Sending 0 byte files now triggers an exception in 3.7.0

[bdraco]

🐞 Describe the bug

#4517 caused a regression with 0 byte files.

💡 To Reproduce

Send a 0 byte file with aiohttp.web.FileResponse on python 3.7 or later

💡 Expected behavior

No exception

📋 Logs/tracebacks

Traceback (most recent call last):
  File "/__w/core/core/venv/lib/python3.7/site-packages/aiohttp/web_protocol.py", line 461, in start
    resp, reset = await task
  File "/__w/core/core/venv/lib/python3.7/site-packages/aiohttp/web_protocol.py", line 414, in _handle_request
    reset = await self.finish_response(request, resp, start_time)
  File "/__w/core/core/venv/lib/python3.7/site-packages/aiohttp/web_protocol.py", line 568, in finish_response
    await prepare_meth(request)
  File "/__w/core/core/venv/lib/python3.7/site-packages/aiohttp/web_fileresponse.py", line 373, in prepare
    return await self._sendfile(request, fobj, offset, count)
  File "/__w/core/core/venv/lib/python3.7/site-packages/aiohttp/web_fileresponse.py", line 191, in _sendfile_system
    await writer.sendfile()
  File "/__w/core/core/venv/lib/python3.7/site-packages/aiohttp/web_fileresponse.py", line 107, in sendfile
    self._count
  File "/usr/local/lib/python3.7/asyncio/base_events.py", line 1044, in sendfile
    offset, count)
  File "/usr/local/lib/python3.7/asyncio/selector_events.py", line 551, in _sendfile_native
    fallback=False)
  File "/usr/local/lib/python3.7/asyncio/base_events.py", line 788, in sock_sendfile
    self._check_sendfile_params(sock, file, offset, count)
  File "/usr/local/lib/python3.7/asyncio/base_events.py", line 842, in _check_sendfile_params
    "count must be a positive integer (got {!r})".format(count))
ValueError: count must be a positive integer (got 0)

📋 Your version of the Python

$ python --version
Python 3.8.1
...

📋 Your version of the aiohttp/yarl/multidict distributions

$ python -m pip show aiohttp
...

$ python -m pip show multidict
...

$ python -m pip show yarl
...

📋 Additional context