Return none if request errors, also black

[jacobtomlinson]

If an API request failed it would never set the response variable but would still try to decode it.

This PR initialises it as None.

Also my editor ran black on the file.

[codecov]

Codecov Report

Merging #7 into master will decrease coverage by 0.22%.
The diff coverage is 10.00%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
- Coverage   42.93%   42.70%   -0.23%     
==========================================
  Files           5        5              
  Lines         191      192       +1     
==========================================
  Hits           82       82              
- Misses        109      110       +1     

Impacted Files	Coverage Δ
opsdroid_homeassistant/connector/__init__.py	32.35% <10.00%> (-0.33%)	⬇️

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