fix: Console error when opening context menu on tree table (#2047)

[AkshatJawne]

Resolves #2029

Changes Implemented:

• Added null check when call formatValue function, to ensure that it is only called when it is defined

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 46.43%. Comparing base (3757163) to head (b710369).
Report is 2 commits behind head on release/v0.78.

Files	Patch %	Lines
packages/iris-grid/src/IrisGridTreeTableModel.ts	0.00%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           release/v0.78    #2142   +/-   ##
==============================================
  Coverage          46.43%   46.43%           
==============================================
  Files                675      675           
  Lines              38841    38841           
  Branches            9830     9831    +1     
==============================================
  Hits               18037    18037           
  Misses             20751    20751           
  Partials              53       53           

Flag	Coverage Δ
unit	46.43% <0.00%> (ø)

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