background_callback: Avoid CALLBACK_CRITICAL_BEGIN with nothing to do
background_callback: Avoid CALLBACK_CRITICAL_BEGIN with nothing to do

CALLBACK_CRITICAL_BEGIN is heavyweight, but we can be confident we do
not have work to do as long as callback_head is NULL.

This gives back performance on nRF.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython