show 'No connection available' in empty site auth

With no LDAP/SAML connection configured, the sub-selector in the site
editor's 'Use the following' authentication connections list rendered
completely blank. Supply the SingleChoiceExtended no_elements_text so
the existing frontend empty-state branch has something to display.
Validation already rejects the empty state and is unchanged.

CMK-37784

Change-Id: Ie2fe895eaea52cf5b8ca3dcbc4d7a3ce5f68a5b0

Author: MakeilaLun

cmk/gui/watolib/sites.py

@@ -506,6 +506,7 @@ def _editable_connections_form_spec(
                 parameter_form=SingleChoiceExtended[str](
                     title=Title("LDAP connection"),
                     elements=ldap_elements,
+                    no_elements_text=Message("No connection available"),
                 ),
             ),
         ]
@@ -528,6 +529,7 @@ def _editable_connections_form_spec(
                                 parameter_form=SingleChoiceExtended[str](
                                     title=Title("SAML connection"),
                                     elements=saml_elements,
+                                    no_elements_text=Message("No connection available"),
                                 ),
                             ),
                             "metadata_endpoint": DictElement(