MLBEDSW-10776 Don't serialize intermediates vector if empty

Most operations do not have any intermediate tensors. In this case there
is no need to create a vector in the flatbuffer file as even empty vectors
contain a 4 byte length header. With this patch the intermediates vector
is only created if any intermediate tensors are present.

Change-Id: I505ebe8c17a577eee2361050d18715207555409d
Signed-off-by: Jacob Bohlin <jacob.bohlin@arm.com>