Fix graceful shutdown

[andreymal]

This line broke the graceful shutdown: https://github.com/huge-success/sanic/pull/1423/files#diff-bd84a6e523017a7f3d6bd72ee689dce2R110

Because a empty_set or set() expression will always create a new set insead of reusing an existing empty set

(The closed HttpProtocol is totally broken and unstable because self.transport is None >_<)

[codecov]

Codecov Report

Merging #1558 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1558   +/-   ##
======================================
  Coverage    91.4%   91.4%           
======================================
  Files          18      18           
  Lines        1804    1804           
  Branches      344     344           
======================================
  Hits         1649    1649           
  Misses        131     131           
  Partials       24      24

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b68a7fe...99e56ef. Read the comment docs.