Fix an exception in notifier when message is empty

If there is an issue sending a notification the callback is invoked with just the error argument and no message. Then in the callback checkActivationNotifierResponse is called with the message argument without checking first if it is valid causing an exception.

Before this change the error shown was: "Problem initializing notification" "TypeError: Cannot read property 'trim' of undefined

The real error was masked by the exception: "Problem with notification" "Error: not found: notify-send"

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Closes https://gitlab.com/gitlab-org/gitter/desktop/issues/185