Allow empty message if from fallback locale

https://github.com/rmariuzzo/Lang.js/blob/1d8e091d429816b4f09882b617603ccdae0d99d7/src/lang.js#L297

If we have an empty value for a given key in a fallback locale:

`
'button.tooltip' => ''
`

and we request _Lang.get('button.tooltip')_ then __getMessage_ returns 'button.tooltip'.

`
if (message[subKey] !== undefined) {
`

solves the problem.