fix: only auto-submit SSO registration form when provider returns an email (#38860)

When a third-party auth provider has "skip registration form" enabled, the
authn MFE context set autoSubmitRegForm=True unconditionally. Providers such
as Facebook (email permission denied) or Microsoft Entra ID can complete
authentication without returning an email claim; auto-submitting the
registration form in that case silently fails client-side validation and
leaves the learner stuck on a partially-filled form with no explanation.

Gate the auto-submit on the provider actually returning an email so the
learner falls back to the normal registration form and can fill in what is
missing.

Fixes: https://github.com/openedx/edx-platform/issues/38780