fix: Change display of rollup key columns from null to empty string (#1563)

* 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 #1483 

#### Testing Instructions:
1. Run:
  ```python
  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

---------

Co-authored-by: georgecwan <georgecwan@users.noreply.github.com>
Co-authored-by: Mike Bender <mikebender@deephaven.io>

Author: georgecwan

packages/iris-grid/src/IrisGridTreeTableModel.ts

@@ -33,6 +33,15 @@ class IrisGridTreeTableModel extends IrisGridTableModelTemplate<
         const value = this.valueForCell(x, y);
         return this.displayString(value, column.constituentType, column.name);
       }
+      // Show empty string instead of null in rollup grouping columns (issue #1483)
+      if (
+        row.hasChildren &&
+        row.depth <= x + 1 &&
+        this.valueForCell(x, y) === null &&
+        this.getCachedGroupedColumnSet(this.groupedColumns).has(x)
+      ) {
+        return '';
+      }
     }
 
     return super.textForCell(x, y);
@@ -182,6 +191,15 @@ class IrisGridTreeTableModel extends IrisGridTableModelTemplate<
       )
   );
 
+  getCachedGroupedColumnSet = memoize(
+    (groupedColumns: readonly Column[]) =>
+      new Set(
+        groupedColumns.map(c1 =>
+          this.columns.findIndex(c2 => c1.name === c2.name)
+        )
+      )
+  );
+
   updateFrozenColumns(columns: ColumnName[]): void {
     if (columns.length > 0) {
       throw new Error('Cannot freeze columns on a tree table');