Don't crash when no dialog title is provided.

[bddckr]

The title property is non-nullable on the native side, so we cannot provide it a null NSString without crashing - the following is printed to the console:

*** Assertion failure in -[NSOpenPanel setTitle:], NSWindow.m:2610

The minimum reproduction was to do the following in the web page:

const dialog = new OpenFileDialog();
const { file } = await dialog.showAsync();