fix: missing uns array returns error instead of None

[kaloster]

This is a fix for get_spatial() to return None instead of e when uns array is missing

Tests:

• Pulled same unaltered dataset from s3 (spatial without uns)

• Ran locally with spatial embedding

• Tested altered (spatial with uns)

• Ran locally with spatial embedding

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 76.57%. Comparing base (2b0f86e) to head (b1593f4).

Files	Patch %	Lines
server/dataset/dataset.py	33.33%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #832   +/-   ##
=======================================
  Coverage   76.57%   76.57%           
=======================================
  Files          88       88           
  Lines        6994     6994           
=======================================
  Hits         5356     5356           
  Misses       1638     1638           

Flag	Coverage Δ
frontend	76.57% <33.33%> (ø)
javascript	76.57% <33.33%> (ø)
smokeTest	∅ <ø> (?)
unitTest	76.57% <33.33%> (ø)

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