Customer: fix __str__ for saved customer without users

[blueyed]

pinax#540

[codecov]

Codecov Report

Merging #112 into next will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             next     #112      +/-   ##
==========================================
+ Coverage   98.98%   98.98%   +<.01%     
==========================================
  Files          33       33              
  Lines        1962     1966       +4     
  Branches      176      178       +2     
==========================================
+ Hits         1942     1946       +4     
  Misses         12       12              
  Partials        8        8

Flag	Coverage Δ
#py27dj111	98.98% <100%> (ø)	⬆️
#py36dj20psql	98.98% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
pinax/stripe/models.py	99.77% <100%> (ø)	⬆️

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