Fix the router for request URIs starting with double slashes

[zoglo]

Description

Fixes the failing E2E test as seen here https://github.com/contao/contao/actions/runs/37078047354/job/111072215440 in contao/contao#10400.

For the //, parse_url() returns false, which crashes the router with a 500 before Contao can get the request in the E2E test.

As always, first commit for the failing regression test -> https://github.com/contao/setup-and-testing/actions/runs/37081299810/job/111082191540

[zoglo]

Using ?: '/' handles both null and false from parse_url() which will make sure that double slashes fall back to /.

[zoglo]

/cc @Toflar =)