[backend] Allow passwords with digits only for initial admin (#6382)

[aHenryJard]

Proposed changes

• force value to be a String
• add some test coverage on initial admin + coverage on the bug reproduction

Related issues

• Initial admin password with digits only fails on first platform start #6382

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 67.71%. Comparing base (b1ee60f) to head (5251576).
Report is 2 commits behind head on master.

Files	Patch %	Lines
...pencti-platform/opencti-graphql/src/domain/user.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6489      +/-   ##
==========================================
+ Coverage   67.66%   67.71%   +0.05%     
==========================================
  Files         532      532              
  Lines       64976    64989      +13     
  Branches     5433     5444      +11     
==========================================
+ Hits        43964    44010      +46     
+ Misses      21012    20979      -33     

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