Update geocoder_api.py to handle optional city, country and postal code

[Hazurl]

As in #77, this PR extends address_with_details:

• city is now optional
• country is now optional
• postal_code has been added
• If nothing is provided (since now all parameters can be optional), raise an error before doing a request that would be rejected
• Treat empty strings as None because here will fail anyway

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.98%. Comparing base (cffef39) to head (0d75cf8).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   92.96%   92.98%   +0.02%     
==========================================
  Files          26       26              
  Lines        1749     1754       +5     
==========================================
+ Hits         1626     1631       +5     
  Misses        123      123              

Flag	Coverage Δ
unittests	92.98% <100.00%> (+0.02%)	⬆️

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[abdullahselek]

Thanks for the PR! Looks good to me.