Update icon theme fallback to use default theme (#38485)

https://github.com/zed-industries/zed/pull/38367 introduced panic:

```
thread 'main' panicked at crates/theme/src/settings.rs:812:18:
called `Option::unwrap()` on a `None` value
```

In this PR I restored fallback logic from the original code - before
settings refactor.

Release Notes:

- N/A

crates/theme/src/settings.rs

@@ -808,7 +808,7 @@ impl settings::Settings for ThemeSettings {
             theme_overrides: HashMap::default(),
             active_icon_theme: themes
                 .get_icon_theme(icon_theme_selection.icon_theme(*system_appearance))
-                .ok()
+                .or_else(|_| themes.default_icon_theme())
                 .unwrap(),
             icon_theme_selection: Some(icon_theme_selection),
             ui_density: content.ui_density.unwrap_or_default().into(),