Desktop: Fixes #10082: Fix text not shown in plugin message boxes

[personalizedrefrigerator]

Summary

Fixes an issue where the default message in showMessageBox's options would override the message parameter, resulting in an empty alert dialog.

Fixes #10082.

Testing

This has been manually tested by:

1. Right-click deleting a notebook
2. Verifying that the "Move notebook ... to the trash" dialog appears and can be canceled.
3. Running await joplin.views.dialogs.showMessageBox("Foo") from the development tools for a development plugin.
4. Verifying that the message box that appears has text.