fix: Change display of rollup key columns from null to empty string

[georgecwan]

• Previously, key columns containing non-numeric types would display null is the rolled up rows have different values for the same column
  • This behaviour is updated to show an empty cell instead of null
• Resolves Rollup grouping columns shouldn't show as 'null' #1483

Testing Instructions:

1. Run:

from deephaven import empty_table, time_table

size = 100
scale = 1000

random_values_with_null_NaN = empty_table(size).update([
    "MyString=(i%11==0? null : `a`+(int)(scale*(Math.random()*2-1)))"
])

2. Add the string and char columns and any other arbitrary columns as rollup columns in any order
3. The key column should only show null if it actually represents a null value in the column

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (5592627) 46.43% compared to head (eabc635) 46.46%.
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1563      +/-   ##
==========================================
+ Coverage   46.43%   46.46%   +0.03%     
==========================================
  Files         564      571       +7     
  Lines       35741    36020     +279     
  Branches     8928     9017      +89     
==========================================
+ Hits        16597    16738     +141     
- Misses      19094    19230     +136     
- Partials       50       52       +2     

Flag	Coverage Δ
unit	46.46% <35.71%> (+0.03%)	⬆️

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Files	Coverage Δ
packages/iris-grid/src/PartitionSelectorSearch.tsx	69.75% <81.25%> (-4.11%)	⬇️
...ckages/iris-grid/src/IrisGridPartitionSelector.tsx	34.48% <22.22%> (-0.97%)	⬇️
packages/iris-grid/src/IrisGrid.tsx	26.84% <0.00%> (-0.13%)	⬇️
packages/iris-grid/src/IrisGridTreeTableModel.ts	0.00% <0.00%> (ø)

... and 49 files with indirect coverage changes

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