fix: get default workspace if exits when creating default user

[frascuchon]

Description

This PR fixes the create_default CLI command errors when creating a default user and the default workspace already exists.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.24%. Comparing base (3032d3a) to head (84b20ff).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5558      +/-   ##
===========================================
+ Coverage    91.22%   91.24%   +0.01%     
===========================================
  Files          145      145              
  Lines         5972     5973       +1     
===========================================
+ Hits          5448     5450       +2     
+ Misses         524      523       -1     

Flag	Coverage Δ
argilla-server	91.24% <100.00%> (+0.01%)	⬆️

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