fix: DH-21696: Fixed broken frozen columns

[bmingles]

DH-21696: A default prop value was changed from null to EMPTY_ARRAY in https://github.com/deephaven/web-client-ui/pull/2527/changes#diff-ba5d5bc0271a84729ec1bd28c11b7e1f05b3e71e4015c0bea863b4bf52b85bd8R544 . This was breaking a coalesce in IrisGridTableModel.getMemoizedFrozenColumns(). Fix was to set default to undefined

Testing

Freezing columns via dh.ui should actually freeze the columns:

from deephaven import ui
import deephaven.plot.express as dx

t1 = dx.data.stocks()

t2 = ui.table(
    t1,
    frozen_columns=["Sym", "Exchange"]
)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.51%. Comparing base (a0ef312) to head (696fa21).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2649      +/-   ##
==========================================
- Coverage   49.77%   49.51%   -0.27%     
==========================================
  Files         774      774              
  Lines       43860    43871      +11     
  Branches    11291    11109     -182     
==========================================
- Hits        21832    21723     -109     
- Misses      22010    22130     +120     
  Partials       18       18              

Flag	Coverage Δ
unit	49.51% <ø> (-0.27%)	⬇️

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