Fix error when subclassing rez.command.Command caused by accessing a non-existent config entry

[predat]

extension have been replace by command

[codecov]

Codecov Report

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

Project coverage is 58.39%. Comparing base (e215a77) to head (7b945c8).
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/rez/command.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1788   +/-   ##
=======================================
  Coverage   58.39%   58.39%           
=======================================
  Files         126      126           
  Lines       17205    17205           
  Branches     3519     3519           
=======================================
  Hits        10047    10047           
  Misses       6491     6491           
  Partials      667      667           

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[JeanChristopheMorinPerso]

Thanks @predat! Would you be willing to add a test? It should be relatively straight forward.