backends/ldap: set proper default for iterable and strings (#23698)

src/authentic2/backends/ldap_backend.py

@@ -216,20 +216,22 @@ class LDAPUser(get_user_model()):
 
 class LDAPBackend(object):
     _DEFAULTS = {
-        'binddn': None,
-        'bindpw': None,
+        'basedn': '',
+        'binddn': '',
+        'bindpw': '',
         'bindsasl': (),
-        'user_dn_template': None,
+        'url': (),
+        'user_dn_template': '',
         'user_filter': 'uid=%s',  # will be '(|(mail=%s)(uid=%s))' if A2_ACCEPT_EMAIL_AUTHENTICATION is set (see update_default)
-        'sync_ldap_users_filter': None,
-        'user_basedn': None,
-        'group_dn_template': None,
-        'member_of_attribute': None,
+        'sync_ldap_users_filter': '',
+        'user_basedn': '',
+        'group_dn_template': '',
+        'member_of_attribute': '',
         'group_filter': '(&(member={user_dn})(objectClass=groupOfNames))',
         'group': None,
-        'groupsu': None,
-        'groupstaff': None,
-        'groupactive': None,
+        'groupsu': (),
+        'groupstaff': (),
+        'groupactive': (),
         'group_mapping': (),
         'group_to_role_mapping': (),
         'replicas': True,