Fix test commands defined as list that result in an empty command

[JeanChristopheMorinPerso]

Fixes #1525

Fix test commands defined as list that result in an empty command when verbosity is enabled.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 58.58%. Comparing base (102c8e7) to head (484d625).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/rez/package_test.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1850   +/-   ##
=======================================
  Coverage   58.58%   58.58%           
=======================================
  Files         126      126           
  Lines       17207    17207           
  Branches     3014     3015    +1     
=======================================
  Hits        10080    10080           
  Misses       6474     6474           
  Partials      653      653           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.