Attempt to detect empty SA and skip them

It would seem that in sa_build_layouts, we end up at:

            printf("ZFS: NULL SA: SA_COPY_DATA(%p, %p, %p, %u) i=%u\n",
                   attr_desc[i].sa_data_func, attr_desc[i].sa_data,
                   data_start, length, i);
            SA_COPY_DATA(attr_desc[i].sa_data_func, attr_desc[i].sa_data,
                         data_start, length);

ZFS: SA_COPY_DATA(0, 0xffffff8204d7cb70, 0xffffff82025d4e70, 16) i=10
ZFS: SA_COPY_DATA(0, 0xffffff8204d7cb80, 0xffffff82025d4e80, 8) i=11
ZFS: SA_COPY_DATA(0, 0, 0xffffff82025d4e88, 16) i=12

So clearly attr_desc[12].sa_data is NULL.