py/emitnative: Use NULL for pending exception (not None).
py/emitnative: Use NULL for pending exception (not None).

This previously made the native emitter incompatible with the bytecode
emitter, and mp_resume (and subsequently mp_obj_generator_resume) expects
the bytecode emitter behavior (i.e. throw==NULL).

tobbad/micropython,tobbad/micropython,MrSurly/micropython,henriknelson/micropython,tobbad/micropython,henriknelson/micropython,MrSurly/micropython,henriknelson/micropython,pramasoul/micropython,bvernoux/micropython,adafruit/circuitpython,tobbad/micropython,MrSurly/micropython,MrSurly/micropython,adafruit/circuitpython,adafruit/circuitpython,tobbad/micropython,selste/micropython,selste/micropython,pozetroninc/micropython,bvernoux/micropython,selste/micropython,bvernoux/micropython,pramasoul/micropython,MrSurly/micropython,adafruit/circuitpython,pozetroninc/micropython,bvernoux/micropython,kerneltask/micropython,kerneltask/micropython,pramasoul/micropython,selste/micropython,adafruit/circuitpython,pozetroninc/micropython,pozetroninc/micropython,bvernoux/micropython,pramasoul/micropython,kerneltask/micropython,henriknelson/micropython,henriknelson/micropython,pramasoul/micropython,pozetroninc/micropython,kerneltask/micropython,kerneltask/micropython,adafruit/circuitpython,selste/micropython