Desktop: Fixes #10828: Fix error when canceling bulk PDF export

[personalizedrefrigerator]

Summary

This pull request fixes #10828 — bridge().showOpenDialog() returns string|string[]. When no output paths were selected, it returned an empty array ([]).

Existing code assumed that bridge().showOpenDialog() always returns a string.

Testing plan

1. Select two notes.
2. Click "Export" > "PDF".
3. Click "Cancel".
4. Verify that no error dialog is shown.
5. Select the same two notes.
6. Click "Export", then "PDF".
7. Select an empty output directory.
8. Click "Open".
9. Verify that the directory now contains two PDF files.

This has been tested successfully on Ubuntu 24.04.