Add ManifestStore support for empty chunks.

[sharkinsspatial]

• Tests added
• Tests passing
• Full type hint coverage

Add support for ManifestStore to return None when reading "empty" chunks.

Closes #16

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.34%. Comparing base (7dabefe) to head (80f0589).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #668      +/-   ##
===========================================
+ Coverage    78.30%   78.34%   +0.03%     
===========================================
  Files           34       34              
  Lines         1793     1796       +3     
===========================================
+ Hits          1404     1407       +3     
  Misses         389      389              

Files with missing lines	Coverage Δ
virtualizarr/manifests/manifest.py	86.80% <100.00%> (+0.09%)	⬆️
virtualizarr/manifests/store.py	88.72% <100.00%> (+0.17%)	⬆️

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[maxrjones]

All the failures are related to #673 so I'll go ahead and merge this.