fix: Invalid migration of legacy partitions

[mofojed]

• We didn't check if the partition was null before adding it to an array
• Was setting a partition of [null] for all migrated tables
• Fix up unit tests to actually check the results instead of just checking against the object passed in

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.11%. Comparing base (29353c2) to head (2a3ebd2).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1892      +/-   ##
==========================================
- Coverage   46.12%   46.11%   -0.02%     
==========================================
  Files         635      637       +2     
  Lines       38047    38056       +9     
  Branches     9617     9619       +2     
==========================================
  Hits        17549    17549              
- Misses      20445    20454       +9     
  Partials       53       53              

Flag	Coverage Δ
unit	46.11% <100.00%> (-0.02%)	⬇️

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