proto_read_attribute_array: When count is zero, make result a NULL ptr.

I am a bit uncertain about this change, since both NULL and non-NULL
will make test cases fail within the SoftHSM test suite for example
(some SoftHSM C_ functions want the pointer to be NULL when there
are no attributes, and some allow (expect) it to be possible to pass a
non-NULL pointer with a zero count).

It seems that making it a NULL pointer when the count is 0 is the most
sensible thing though. How could the C_ function use the data pointed
to by the pointer, when the count says there is no data there? The
result would really be undefined.