Add support for empty constant tensor in delegate

The constant tensor's data pointer was reused for the scratch tensors to
avoid TFLite failing because TFLite sees tensors with a size but no data
as an error.

However, in some cases, the constant tensor will be empty and cannot be
reused for the scratch tensors. To avoid this, the command tensor's data
pointer is reused instead as it will always be set.

Change-Id: I40fe5bde04f3cc5cccde96d6e4c1ea2ed2a60e2c
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>