Fix dialog.save return type (#5373)

Co-authored-by: Fabian-Lars <fabianlars@fabianlars.de>

.changes/dialog-save-return-fix.md

@@ -0,0 +1,5 @@
+---
+"api": patch
+---
+
+Fix `dialog.save` return type

tooling/api/src/dialog.ts

@@ -189,7 +189,7 @@ async function open(
  *
  * @since 1.0.0
  */
-async function save(options: SaveDialogOptions = {}): Promise<string> {
+async function save(options: SaveDialogOptions = {}): Promise<string | null> {
   if (typeof options === 'object') {
     Object.freeze(options)
   }