update stac_io from owner root

[KennSmithDS]

Related Issue(s): #742

Description:

Updated edge case for when stac_io is None, owner is not None and instance is not pystac.Catalog. Desired behavior now fetches the root of the owner to update the stac_io of a Link object.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• [N/A] Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[codecov-commenter]

Codecov Report

Merging #762 (c982679) into main (d70dea5) will decrease coverage by 0.00%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main     #762      +/-   ##
==========================================
- Coverage   94.55%   94.54%   -0.01%     
==========================================
  Files          79       79              
  Lines       11622    11626       +4     
  Branches     1370     1372       +2     
==========================================
+ Hits        10989    10992       +3     
  Misses        455      455              
- Partials      178      179       +1     

Impacted Files	Coverage Δ
pystac/link.py	90.81% <75.00%> (-0.35%)	⬇️

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