Verify empty observation direction property

[l0b0]

Related Issue(s): #466

Description:

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.

[codecov-commenter]

Codecov Report

Merging #488 (431b5a7) into main (f9573f8) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #488      +/-   ##
==========================================
+ Coverage   91.66%   91.72%   +0.05%     
==========================================
  Files          40       40              
  Lines        5245     5245              
==========================================
+ Hits         4808     4811       +3     
+ Misses        437      434       -3     

Impacted Files	Coverage Δ
pystac/extensions/sar.py	98.96% <ø> (+1.03%)	⬆️
pystac/extensions/view.py	98.38% <ø> (+0.80%)	⬆️

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