fix: create the brand fallback for {} PARAGON_THEME_URLS

[dcoa]

Description:
If the brand package is installed with a compatible version of design tokens and define PARAGON_THEMES_URLS={} to use the local files, this one is not creating the corresponding urls - only the Paragon default ones. This PR aims to add the logic to permit the creation of those urls.

I added some improvements in the test file based on the comments of the original PR #689 (comment)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.78%. Comparing base (e0656d1) to head (b333419).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #785      +/-   ##
==========================================
+ Coverage   86.71%   86.78%   +0.06%     
==========================================
  Files          48       48              
  Lines        1393     1400       +7     
  Branches      293      297       +4     
==========================================
+ Hits         1208     1215       +7     
  Misses        172      172              
  Partials       13       13              

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[brian-smith-tcril]

LGTM!

Left a small comment with a clarifying question, but it's definitely non-blocking.

[brian-smith-tcril]

Just to make sure I'm understanding this:

• Before this we were assuming we had PARAGON_THEME but not assuming we had PARAGON_THEME.paragon
• With this we are not assuming we have PARAGON_THEME

This is to handle PARAGON_THEME being undefined (as it is with the test change) correct?

[dcoa]

This is to handle PARAGON_THEME being undefined (as it is with the test change) correct?

Yes, it is