Empty log messages generate exceptions in the Notifier class

Rails likes to log empty lines, particularly ActionView. Unfortunately that triggers an exception in the `Notifier#check_presence_of_mandatory_attributes` method because the `short_message` value is an empty string.
This change ensures that messages send to the Notifier always have a non-empty `short_message` field. Empty log lines and hashes without a `short_message` value will be ignored.
This change also allows to specify the `facility` in the log message if it is a hash which is useful when used in combination with other logging frameworks or even in your logging own code.