iv-gen: Use NULL IV generator for NULL encryption

iv-gen: Use NULL IV generator for NULL encryption

We don't need an IV for NULL encryption, so we wouldn't technically need
an IV generator.  But some of the code currently relies on an IV
generator to be present.  So we don't have to change that code and
handle IV size == 0 specially we use the new NULL IV generator, which
handles this transparently to the existing code.

Before 3c81cb6fc322 ("aead: Create AEAD using traditional transforms
with an explicit IV generator") iv_gen_rand_t was used for NULL
encryption, which would work too but this way it's clearer.