MLBEDSW-2874 Fix writing out empty tflite files

If a tflite file with no ops but just the input/output tensor is given,
vela wrote an empty optimised tflite file with no tensors given.
This fixes that by allowing all placeholder tensors to also be
serialised on write.

Signed-off-by: Michael McGeagh <michael.mcgeagh@arm.com>
Change-Id: If79817100869e712a75264889f401e38de0b1e7a