[data grid] Do not fallback on valueToRender when children === null

[joserodolfofreitas]

Summary

In GridCell, do not fallback on valueToRender when children === null but only when children === undefined

mui-x/packages/grid/x-data-grid/src/components/cell/GridCell.tsx

Lines 222 to 224 in b778028

	if (children == null) {
	return <div className={classes.content}>{valueToRender?.toString()}</div>;
	}

Motivation

DX: To allow to return null in render cells.