Only assert valid next_link params when provided (#8417)

Broken in https://github.com/matrix-org/synapse/pull/8275 and has yet to be put in a release. Fixes https://github.com/matrix-org/synapse/issues/8418.

`next_link` is an optional parameter. However, we were checking whether the `next_link` param was valid, even if it wasn't provided. In that case, `next_link` was `None`, which would clearly not be a valid URL.

This would prevent password reset and other operations if `next_link` was not provided, and the `next_link_domain_whitelist` config option was set.