spi: rtio: Add validation for NULL tx and rx buffers

In addition to #94710 fixes, SPI RTIO currently fails existing spi
loopback testcase: test_spi_null_tx_rx_buf_set.

This plus the referenced PR enable RTIO-enabled drivers to pass the
testsuite.

Signed-off-by: Luis Ubieda <luisf@croxel.com>