fix: don't throw when history.state is null after replaceState

[tomheaton]

saveCurrentDepth() runs on import and reads window.history.state._depth right after replaceState. In an iOS WKWebView (a Flutter flutter_inappwebview embed in our case) replaceState doesn't throw but history.state stays null, so importing the router throws:

TypeError: null is not an object (evaluating 'window.history.state._depth')

Because it happens at module eval, the whole app bundle fails to load, even when only MemoryRouter is used.

This falls back to history.length - 1, the same value it tries to write. Nothing changes when history.state works.

Added a test that mocks history.state as null and replaceState as a no-op. It fails without the fix.

Same fix for next: #627

[changeset-bot]

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Latest commit: 58e2d41

The changes in this PR will be included in the next version bump.

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Name	Type
@solidjs/router	Patch

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