fix: support not setting default in variant when using PARAGON_THEME_URLS

[dcoa]

Description:

This PR aims to allow the creation of URLs with only brandOverride definition.

Solving the following use case #808

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.79%. Comparing base (9f92281) to head (c9cd411).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #810   +/-   ##
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  Coverage   86.78%   86.79%           
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  Files          48       48           
  Lines        1400     1401    +1     
  Branches      296      297    +1     
=======================================
+ Hits         1215     1216    +1     
  Misses        172      172           
  Partials       13       13           

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[brian-smith-tcril]

Sorry this took a bit to review. useParagonThemeUrls is quite big so I wanted to be thorough and properly think through all the possible implications of these changes.

Thank you so much for this! This will be a wonderful QoL improvement for theme authors!