Fix email registration, pt. 2
Regressed in https://github.com/matrix-org/matrix-react-sdk/pull/2768
where we check for an existing stored account first and restore that
instead if it exist, telling the user. We usually make a guest account
when the user first hits the page though, so this just restored this
guest account.
Don't restore the account if it's just a guest account (which, as per
comment, is not perfect, but is definitely better than the current
behaviour).
Fixes https://github.com/vector-im/riot-web/issues/9581
