fix: DH-21696: Fixed broken frozen columns (#2649)

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:
```py
from deephaven import ui
import deephaven.plot.express as dx

t1 = dx.data.stocks()

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

Author: bmingles

packages/iris-grid/src/IrisGrid.tsx

@@ -554,7 +554,7 @@ class IrisGrid extends Component<IrisGridProps, IrisGridState> {
     },
     canCopy: true,
     canDownloadCsv: true,
-    frozenColumns: EMPTY_ARRAY,
+    frozenColumns: undefined,
     // Do not set a default density prop since we need to know if it overrides the global density setting
     density: undefined,
     canToggleSearch: true,