MMS: Fix prompt "message size limit reached" when send 0KB picture

There is no data in this image, when user send this image,
it will transfer to a pdu which content is null. It will throw
a ExceedMessageSizeException and alert user "message size limit
reached ".

Check the image whether is avialable when we create ImageModel.

CRs-Fixed: 413024

Change-Id: I33d8d1332ab6995b7d1a7a33cb5cab62168322f2