fix: fix a popState race-condition

When loading a non-SSR page and pressing the browser's back button while
the page is still loading, it's possible for `current.url` to still be
the default value of `null`, which then causes the `popState` function
to throw the following uncaught error:

```
Uncaught (in promise) TypeError: Cannot destructure property 'href' of 'object null' as it is null.
    at strip_hash (url.js?v=3a24e001:84:30)
    at client.js?v=3a24e001:2227:52
```

This is from
https://github.com/sveltejs/kit/blob/e41a8e24b71cb9e9ee249ddfe732a3d0721d40bf/packages/kit/src/runtime/client/client.js#L2234

Author: aloisklink

.changeset/angry-months-speak.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: fix race-condition when not using SSR when pressing back before initial load

packages/kit/src/runtime/client/client.js

@@ -2231,7 +2231,7 @@ function _start_router() {
 			const state = event.state[STATES_KEY] ?? {};
 			const url = new URL(event.state[PAGE_URL_KEY] ?? location.href);
 			const navigation_index = event.state[NAVIGATION_INDEX];
-			const is_hash_change = strip_hash(location) === strip_hash(current.url);
+			const is_hash_change = current.url ? strip_hash(location) === strip_hash(current.url) : false;
 			const shallow =
 				navigation_index === current_navigation_index && (has_navigated || is_hash_change);