fix(lint): better undefined typing

Co-authored-by: Pytal <24800714+Pytal@users.noreply.github.com>
Signed-off-by: John Molakvoæ <skjnldsv@users.noreply.github.com>

lib/files/node.ts

@@ -108,14 +108,14 @@ export abstract class Node {
 	/**
 	 * Get the file modification time
 	 */
-	get mtime(): Date|undefined {
+	get mtime(): ?Date {
 		return this._data.mtime
 	}
 
 	/**
 	 * Get the file creation time
 	 */
-	get crtime(): Date|undefined {
+	get crtime(): ?Date {
 		return this._data.crtime
 	}