drivers: spi_nrfx_spis: Handle empty spi_buf_set structures properly

SPI API allows `spi_buf_set` structures with no buffers linked to them
(with `.buffers = NULL`). Correct the spi_nrfx_spis driver so that it
is able to deal with such structures.

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