Update unit test mock return type

Author: bvaughn

lib/components/grid/Grid.test.tsx

@@ -168,7 +168,7 @@ describe("Grid", () => {
       />
     );
 
-    const NewCellComponent = vi.fn(() => null);
+    const NewCellComponent = vi.fn(() => <div />);
 
     rerender(
       <Grid

lib/components/list/List.test.tsx

@@ -181,7 +181,7 @@ describe("List", () => {
       />
     );
 
-    const NewRowComponent = vi.fn(() => null);
+    const NewRowComponent = vi.fn(() => <div />);
 
     rerender(
       <List