fix: ENT-11771 SAML login - Allow email editing if emtpy

[jono-booth]

Small follow up to make email editable if blank.
ENT-11771

[Copilot]

Pull request overview

This PR adjusts SAML SSO registration behavior so that disable_email_editing=True only makes the email field read-only when the SAML pipeline actually provides a non-empty email value—allowing users to enter an email when the IdP doesn’t supply one.

Changes:

• Gate the “email readonly” override behind the presence of a non-empty email value from get_register_form_data.
• Add a regression test ensuring the email field remains editable when no email is provided by the pipeline (even if disable_email_editing=True).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
openedx/core/djangoapps/user_authn/views/registration_form.py	Only applies the email readonly restriction when the provider-supplied email is non-empty.
openedx/core/djangoapps/user_authn/views/tests/test_saml_disable_email_editing.py	Adds coverage for the “disable editing but missing email” scenario to prevent regressions.

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