Fix problem passing null optional operands.

Allow optional operands to be specified as having no value in the model.
Make sure that both these and model arguements that have no value can
be passed to through the HAL. Change the validation to ensure that all
required ops are there.

Bug: 63905942
Test: Compiled and ran runtime/test and VTS tests.

Change-Id: Ia20954b9712042f5c0571822967521aedb66150b