MLBEDSW-7373: Vela sometimes write empty buffers in incorrect format

 - Fixed an issue whereby a zero length buffer was written out instead
of an empty buffer
 - Added a warning message to highlight when this type of semantically
incorrect empty buffer is read from an input network

Change-Id: Iac3bc71a2dbfda53737bbeb6e7f895552f0f13d0
Signed-off-by: Tim Hall <tim.hall@arm.com>