BACKPORT Improve regform email validation RH (indico#6275)

Fail nicely in case of empty email address

indico/modules/events/registration/controllers/display.py

@@ -32,7 +32,7 @@
 from indico.modules.users.util import send_avatar, send_default_avatar
 from indico.util.fs import secure_filename
 from indico.util.i18n import _
-from indico.util.marshmallow import UUIDString
+from indico.util.marshmallow import UUIDString, not_empty
 from indico.web.args import use_kwargs
 from indico.web.flask.util import send_file, url_for
 
@@ -258,7 +258,7 @@ class RHRegistrationFormCheckEmail(RHRegistrationFormBase):
     ALLOW_PROTECTED_EVENT = True
 
     @use_kwargs({
-        'email': fields.String(required=True),
+        'email': fields.String(required=True, validate=not_empty),
         'update': UUIDString(load_default=None),
         'management': fields.Bool(load_default=False),
     }, location='query')