register zimfarm worker without email

[elfkuzco]

Rationale

openzim/zimfarm#1720 propoes to drop email column in DB. This PR removes the user email during registration of worker account to signify that emails aren't used during registration anymore in zimfarm.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.78%. Comparing base (7309ed5) to head (8a998c1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1088   +/-   ##
=======================================
  Coverage   92.78%   92.78%           
=======================================
  Files          74       74           
  Lines        4308     4308           
=======================================
  Hits         3997     3997           
  Misses        311      311           

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