Customer: fix __str__ for saved customer without users

[blueyed]

No description provided.

[codecov]

Codecov Report

Merging #540 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #540      +/-   ##
=========================================
+ Coverage    99.4%   99.4%   +<.01%     
=========================================
  Files          33      33              
  Lines        1838    1842       +4     
  Branches      168     170       +2     
=========================================
+ Hits         1827    1831       +4     
  Misses          5       5              
  Partials        6       6

Flag	Coverage Δ
#py27dj110	99.07% <100%> (ø)	⬆️
#py27dj111	99.07% <100%> (ø)	⬆️
#py27dj18	99.34% <100%> (ø)	⬆️
#py34dj110	99.07% <100%> (ø)	⬆️
#py34dj111	99.07% <100%> (ø)	⬆️
#py34dj18	99.34% <100%> (ø)	⬆️
#py34dj20	99.07% <100%> (ø)	⬆️
#py35dj110	99.07% <100%> (ø)	⬆️
#py35dj111	99.07% <100%> (ø)	⬆️
#py35dj18	99.34% <100%> (ø)	⬆️
#py35dj20	99.07% <100%> (ø)	⬆️
#py36dj111	99.07% <100%> (ø)	⬆️
#py36dj20	99.07% <100%> (ø)	⬆️
#py36dj20psql	99.07% <100%> (ø)	⬆️

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

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