router: Fix router forgetting about state (fixes #2164)

[luxalpa]

I am pulling the "State" field in a similar manner to the other location fields within BrowserIntegration::current(). I also changed the State serialization for None from UNDEFINED to NULL as the latter is the default in browsers and should be more consistent. I am not sure what the reason was to originally use undefined and State(None).

[gbj]

Thanks! I've never used state myself, it was just part of the original port of the router, so I can guarantee there was not a good reason. Most of the state stuff was // TODO for a long time so apologies if you run into additional issues.

[gbj]

Failing tests are due to new clippy warnings outside this PR caused by new nightly clippy. I addressed those in #2169 — would you mind rebasing onto main so the CI will run again?