fix: DH-17199: Filter by value in the tree table context menu always shows null

[vbabich]

Provide an initializer for the optional formatValue argument in IrisGridTreeTableModel snapshot method.
Remove the conditional that caused the resultRow to be an empty array when the formatValue argument isn't provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 46.60%. Comparing base (640e002) to head (05ed7ca).

Files	Patch %	Lines
packages/iris-grid/src/IrisGridTreeTableModel.ts	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2078      +/-   ##
==========================================
- Coverage   46.60%   46.60%   -0.01%     
==========================================
  Files         677      677              
  Lines       38564    38565       +1     
  Branches     9769     9769              
==========================================
  Hits        17974    17974              
- Misses      20538    20539       +1     
  Partials       52       52              

Flag	Coverage Δ
unit	46.60% <0.00%> (-0.01%)	⬇️

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