Return none on empty values

[klaasnicolaas]

This PR will do:

• If the value is "" return a None
• Update the test models
• 📚 Update the documentation about Poetry

[codecov]

Codecov Report

Merging #30 (58ae541) into master (e356b6a) will increase coverage by 0.42%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   90.55%   90.97%   +0.42%     
==========================================
  Files           4        4              
  Lines         127      133       +6     
  Branches       17       19       +2     
==========================================
+ Hits          115      121       +6     
  Misses          9        9              
  Partials        3        3              

Impacted Files	Coverage Δ
omnikinverter/omnikinverter.py	78.94% <ø> (ø)
omnikinverter/models.py	100.00% <100.00%> (ø)

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