fix(reana-admin): skip retention rules with deleted workspace (#697)

[mdonadoni]

Closes #693

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 60.84%. Comparing base (3596ee0) to head (d1cd1a5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
reana_server/reana_admin/cli.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #697      +/-   ##
==========================================
- Coverage   60.87%   60.84%   -0.04%     
==========================================
  Files          33       33              
  Lines        3458     3463       +5     
==========================================
+ Hits         2105     2107       +2     
- Misses       1353     1356       +3     

Files with missing lines	Coverage Δ
reana_server/reana_admin/cli.py	51.79% <40.00%> (-0.12%)	⬇️

[tiborsimko]

Works nicely 👍

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fix(reana-admin): skip retention rules for deleted workspaces (#697)