feat: Differentiate between missing column and column with null/NaN values

[ejmolinelli]

Reason for Change

• Validator outputs misleading errors when NaN values are in required Visium .obs columns #1196

Changes

• split out check on validating tissue_position_column into a) test for column existence, and (b) test for null/NaN values.

Testing

• nothing new

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.89%. Comparing base (54bc35d) to head (9c2c812).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1230      +/-   ##
==========================================
+ Coverage   89.83%   89.89%   +0.06%     
==========================================
  Files          19       19              
  Lines        2184     2188       +4     
==========================================
+ Hits         1962     1967       +5     
+ Misses        222      221       -1     

Components	Coverage Δ
cellxgene_schema_cli	90.85% <100.00%> (+0.08%)	⬆️
migration_assistant	91.26% <ø> (ø)
schema_bump_dry_run_genes	79.80% <ø> (ø)
schema_bump_dry_run_ontologies	99.53% <ø> (ø)