Add missing default to preferences

aleksis/apps/matrix/preferences.py

@@ -13,6 +13,7 @@ class Homeserver(StringPreference):
     section = matrix
     name = "homeserver"
     verbose_name = _("URL of Matrix homeserver")
+    default = ""
 
 
 @site_preferences_registry.register
@@ -20,3 +21,4 @@ class AccessToken(StringPreference):
     section = matrix
     name = "access_token"
     verbose_name = _("Access token to access homeserver")
+    default = ""