drivers: spi_context: Fix handling of zero-length buffers

In some cases, it is quite useful to have the possibility to also
include zero-length buffers in a buffer set used in transfers
(for example, when frames in a protocol consist of several parts,
of which some are optional). So far, the behavior of spi_context
update functions was that the transfer in a given direction was
finished when a zero-length buffer was encountered in the buffer
set. Change those functions to simply skip such buffers. Correct
in the same way also the spi_context_buffers_setup() function.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>