login form: make onOriginSubmitted a required callback

login form: make onOriginSubmitted a required callback

as an user can hit enter on the field to start a submission even if
the submit button is disabled, e.g., due to no origin value being
entered at all yet.

Basically we already had that as contract, as forced dereferencing
via ! for accessing the callback.

As the callback already checks the validation state itself we do not
need to change anything there.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>