Omitting `next_link` parameter while using the `next_link_domain_whitelist` option will prevent 3PID validation

[anoadragon453]

#8275 introduced the new config option next_link_domain_whitelist, which allows a server admin to only allow a specific set of domains to be used for next_link functionality while validating 3PID addresses.

There was an oversight however, in that the PR assumed next_link would always be provided, and thus always need to be validated. next_link is an optional parameter though, meaning the request would fail if this config option were in use, and next_link was not provided.

This config option and bug have not shipped in a public release yet, hence the release-blocker tag.