fix: default e-mail "" was invalid and should be None

[tomasvotava]

Closes #165

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.78%. Comparing base (8ae7c5d) to head (1d83eaa).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #166   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
=======================================
  Files          20       20           
  Lines         479      479           
=======================================
  Hits          454      454           
  Misses         25       25           

Files	Coverage Δ
fastapi_sso/sso/facebook.py	100.00% <100.00%> (ø)
fastapi_sso/sso/google.py	61.11% <ø> (ø)
fastapi_sso/sso/spotify.py	100.00% <ø> (ø)