fix issue with subscribe/psubscribe with empty pool (fixes #351)

[popravich]

No description provided.

[codecov]

Codecov Report

Merging #355 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #355      +/-   ##
==========================================
+ Coverage   96.86%   97.04%   +0.17%     
==========================================
  Files          54       54              
  Lines        7309     7343      +34     
  Branches      519      519              
==========================================
+ Hits         7080     7126      +46     
+ Misses        168      161       -7     
+ Partials       61       56       -5

Impacted Files	Coverage Δ
aioredis/commands/pubsub.py	100% <100%> (ø)	⬆️
tests/pubsub_commands_test.py	100% <100%> (ø)	⬆️
aioredis/sentinel/pool.py	84.03% <0%> (+4.56%)	⬆️

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