fix(macos): avoid panics on missing URL (fix #1752)

[Buncys]

Summary

This PR removes unwrap calls in two code paths, as requested in #1752.

Affected code paths

• src/wkwebview/mod.rs, url_from_webview()
• src/wkwebview/navigation.rs, navigation_policy()

Changes

• url_from_webview() now returns Ok("") when URL is unavailable, which is consistent with other platforms
• Navigation will be canceled instead of panicking, when URL is unavailable in navigation_policy()

Fixes #1752

[sftse]

Fixes to non-trivial issues are nearly impossible to judge without a reproducer.

[Buncys]

Here is the reproducer:

let event_loop = EventLoop::new();
let window = WindowBuilder::new().build(&event_loop).unwrap();
let web_view = WebViewBuilder::new().build(&window).unwrap();
let _ = dbg!(web_view.url()); // <- panics on macos

Running this produces:

thread 'main' panicked at .../wry-0.55.1/src/wkwebview/mod.rs:1349:40:
called `Option::unwrap()` on a `None` value