fix: avoid error when back navigating before page is initialized

[nhunzaker]

Fixes an exception when back navigating before a page is fully initialized. I believe this may also fix #10512.

I'm observing that the popstate event callback raises a null pointer exception when it is triggered before current.url has been set, resulting in the following error:

TypeError
null is not an object (evaluating 'current.url.href')

From what I can follow:

1. The page is loaded
2. The user navigates to a second route
3. The page is reloaded
4. The user immediately navigates back before page initialization executes
5. The navigation event raises an exception comparing the current and next url when restoring scroll position
   

When the exception is raised, the navigation code executed after scroll restoration additionally is not executed, which could be the cause of other issues mentioned in #10512

How to reproduce

I found this was easy to test in an iOS similator:

1. Navigate to your app in the iOS simulator
2. Initiate a web inspector session from Safari (Develop > Simulator > localhost)
3. In the simulator, click to a second page
4. Focus the URL address bar
5. Move your mouse to the back button
6. Press enter to submit the URL input
7. Press the back button with your mouse almost immediately

While this specific case is fairly contrived, I believe it could also be responsible for errors we are seeing as a user navigates back and forward very quickly.

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Latest commit: 06a72d9

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[dummdidumm]

Thank you!