A temporary fix to address issues with Mxit message parsing

If no links are in the message an empty link array is included in the JSON message to Mxit. This causes an empty message to be sent, instead of ignoring the Link field. This has been reported to Mxit. 

This fix removes the Link field when empty and is necessary once commit 83f08a8 (Add link functionality to REST messages) is added.

(added as a separate commit so that it can easily be rolled back once Mxit fixes their side)