修复用户名可能为空的错误，修复后台不能修改密码的问题

[liangliangyy]

No description provided.

[codecov]

Codecov Report

Merging #286 into master will increase coverage by 0.04%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
+ Coverage   81.13%   81.17%   +0.04%     
==========================================
  Files          50       50              
  Lines        2051     2056       +5     
==========================================
+ Hits         1664     1669       +5     
  Misses        387      387

Impacted Files	Coverage Δ
accounts/models.py	100% <100%> (ø)	⬆️
accounts/views.py	80.19% <100%> (+0.99%)	⬆️
accounts/admin.py	71.42% <77.77%> (+0.37%)	⬆️

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[coveralls]

Coverage increased (+0.05%) to 81.177% when pulling 260244c on dev into 0614395 on master.