Minor Stac improvements

[jeremyh]

Address some more edge cases allowed in Stac 1.0.0 and newer Stac API (beta) standards.

Will depend on the updates in opendatacube/eo-datasets#222 too.

(Relevant to #303)

[WIP]

• Document/Test: Users should rerun --init to update their stac indexes (to fit with nullable geometry).

[codecov]

Codecov Report

Merging #351 (6177d39) into develop (1ec509b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 6177d39 differs from pull request most recent head 6994e38. Consider uploading reports for the commit 6994e38 to get more accurate results

@@             Coverage Diff             @@
##           develop     #351      +/-   ##
===========================================
- Coverage    86.47%   86.46%   -0.01%     
===========================================
  Files           25       25              
  Lines         3119     3117       -2     
===========================================
- Hits          2697     2695       -2     
  Misses         422      422              

Impacted Files	Coverage Δ
cubedash/_stac.py	91.63% <ø> (ø)
cubedash/summary/_extents.py	91.83% <ø> (ø)
cubedash/summary/_schema.py	81.51% <ø> (ø)
cubedash/summary/_stores.py	91.52% <100.00%> (-0.04%)	⬇️

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