getPreferredEditorId return undefined instead of null

src/vs/workbench/common/editor.ts

@@ -380,12 +380,12 @@ export abstract class EditorInput extends Disposable implements IEditorInput {
 	 * Returns the preferred editor for this input. A list of candidate editors is passed in that whee registered
 	 * for the input. This allows subclasses to decide late which editor to use for the input on a case by case basis.
 	 */
-	getPreferredEditorId(candidates: string[]): string | null {
+	getPreferredEditorId(candidates: string[]): string | undefined {
 		if (candidates.length > 0) {
 			return candidates[0];
 		}
 
-		return null;
+		return undefined;
 	}
 
 	/**