[fix] Disable PUT for user api, deny empty email

[triklozoid]

No description provided.

[codecov]

Codecov Report

Merging #1513 (b350501) into master (efc000a) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master    #1513   +/-   ##
=======================================
  Coverage   75.55%   75.56%           
=======================================
  Files         124      124           
  Lines        8711     8712    +1     
=======================================
+ Hits         6582     6583    +1     
  Misses       2129     2129           

Impacted Files	Coverage Δ
label_studio/users/api.py	83.95% <50.00%> (+0.20%)	⬆️
label_studio/users/models.py	75.67% <100.00%> (ø)

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