fix(DataTable): Don't cast undefined to a string (#365)

Author: williaster

packages/core/src/components/DataTable/ColumnLabels.tsx

@@ -74,7 +74,7 @@ export default function ColumnLabels({
 
       // TODO: The types here are very wrong and confusing.
       const { children } = col.props;
-      const key = String(children[0].props.children);
+      const key = children[0].props.children ? String(children[0].props.children) : '';
       const label = columnToLabel[key]
         ? columnToLabel[key]
         : key && caseColumnLabel(key, columnLabelCase!);