Adjust `xcm-bridge-hub-router`'s `SendXcm::validate` behavior for `NotApplicable` (#4162)

This PR adjusts `xcm-bridge-hub-router` to be usable in the chain of
routers when a `NotApplicable` error occurs.

Closes: https://github.com/paritytech/polkadot-sdk/issues/4133

## TODO

- [ ] backport to polkadot-sdk 1.10.0 crates.io release