Bug introduced in https://github.com/matrix-org/synapse-dinsic/commit/ff91a451b
We were checking whether the `nextLink` param was valid, even if it wasn't provided. In that case, `nextLink` was `None`, which would clearly not be a valid URL.
This would prevent password reset and other operations if `nextLink` was not provided and the `next_link_domain_whitelist` config option was in use.
