handle null value as gogoproto does while modelizing response data

[Revolution1]

fix #74

[Revolution1]

@pjz

[codecov]

Codecov Report

Merging #87 into master will increase coverage by 0.03%.
The diff coverage is 91.52%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   91.48%   91.52%   +0.03%     
==========================================
  Files          52       52              
  Lines        2866     2878      +12     
  Branches      297      298       +1     
==========================================
+ Hits         2622     2634      +12     
- Misses        140      142       +2     
+ Partials      104      102       -2

Impacted Files	Coverage Δ
tests/test_kv_apis.py	100% <100%> (ø)	⬆️
etcd3/models.py	100% <100%> (ø)	⬆️
etcd3/stateful/watch.py	88.23% <100%> (ø)	⬆️
etcd3/swagger_helper.py	79.6% <100%> (+1.34%)	⬆️
tests/test_swagger_helper.py	100% <100%> (ø)	⬆️
etcd3/utils.py	86.27% <16.66%> (-5.4%)	⬇️

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