Return None from Asset.get_absolute_href when owning Item self HREF is NOne

[duckontheweb]

**Related Issue(s):

• Closes Asset.get_absolute_href() should return None but doesn't #754

Description:

Asset.get_absolute_href now returns None when the owning Item has no self HREF.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• 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.

[duckontheweb]

cc: @Kirill888

[codecov-commenter]

Codecov Report

Merging #808 (ad194a2) into main (3526b65) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #808      +/-   ##
==========================================
+ Coverage   94.31%   94.32%   +0.01%     
==========================================
  Files          83       83              
  Lines       11873    11885      +12     
  Branches     1387     1388       +1     
==========================================
+ Hits        11198    11211      +13     
+ Misses        496      495       -1     
  Partials      179      179              

Impacted Files	Coverage Δ
pystac/asset.py	86.11% <100.00%> (+1.82%)	⬆️
tests/test_item.py	100.00% <100.00%> (ø)

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