ConfigFile.get_value() should accept default_value as 3rd parameter
ConfigFile.get_value() should have a 3rd parameter which will be returned if the section & key does not exist.
Suggested edit:
void get_value( String section, String key, var default_value=null ) const
For example:
var cf = ConfigFile.new()
cf.load("user://configs.cfg")
var user_id = cf.get_value("user", "id", -1)
# if user/id does not exist, user_id = -1
# else user_id = whatever value is saved