models: use blank=True with null=True

[blueyed]

Fixes #474.

[codecov]

Codecov Report

Merging #521 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #521   +/-   ##
=======================================
  Coverage   99.39%   99.39%           
=======================================
  Files          34       34           
  Lines        1817     1817           
  Branches      166      166           
=======================================
  Hits         1806     1806           
  Misses          5        5           
  Partials        6        6

Flag	Coverage Δ
#py27dj110	99.06% <100%> (ø)	⬆️
#py27dj111	99.06% <100%> (ø)	⬆️
#py27dj18	99.33% <100%> (ø)	⬆️
#py34dj110	99.06% <100%> (ø)	⬆️
#py34dj111	99.06% <100%> (ø)	⬆️
#py34dj18	99.33% <100%> (ø)	⬆️
#py34dj20	99.06% <100%> (ø)	⬆️
#py35dj110	99.06% <100%> (ø)	⬆️
#py35dj111	99.06% <100%> (ø)	⬆️
#py35dj18	99.33% <100%> (ø)	⬆️
#py35dj20	99.06% <100%> (ø)	⬆️
#py36dj111	99.06% <100%> (ø)	⬆️
#py36dj20	99.06% <100%> (ø)	⬆️
#py36dj20psql	99.06% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
pinax/stripe/models.py	100% <100%> (ø)	⬆️

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