FIX: Handle empty email address from authentication provider (#12046)

If no email is provided, email_valid should be set false, so that
Discourse can prompt the user for an email and verify it.

This fixes signups via twitter for accounts with no email address.

lib/auth/managed_authenticator.rb

@@ -103,7 +103,7 @@ class Auth::ManagedAuthenticator < Auth::Authenticator
       result.name = nil
     end
     result.username = info[:nickname]
-    result.email_valid = primary_email_verified?(auth_token) if result.email
+    result.email_valid = primary_email_verified?(auth_token) if result.email.present?
     result.extra_data = {
       provider: auth_token[:provider],
       uid: auth_token[:uid]