Special treatment of null qstr means "NULL" is not valid in mpy-cross. #5140
Description
Minimum repro:
Create a file with the contents:
def expode():
return "NULL"
Add this as a frozen file so that it is compiled with mpy-cross and output in to frozen.c.
frozen.c now conflicts with qstr.generated.h with a duplicate MP_QSTR_NULL definition.
Seems to work fine in the REPL, presumably because the string is not QSTRd (interned)
build-wscale/frozen_mpy.c:764:5: error: redeclaration of enumerator 'MP_QSTR_NULL'
MP_QSTR_NULL,
^
In file included from ../../py/obj.h:31:0,
from ../../py/objint.h:30,
from build-wscale/frozen_mpy.c:2:
build-wscale/genhdr/qstrdefs.generated.h:3:6: note: previous definition of 'MP_QSTR_NULL' was here
QDEF(MP_QSTR_NULL, (const byte*)"\x00\x00\x00" "")
^
../../py/qstr.h:41:23: note: in definition of macro 'QDEF'
#define QDEF(id, str) id,
^
../../py/mkrules.mk:47: recipe for target 'build-wscale/build-wscale/frozen_mpy.o' failed
make: *** [build-wscale/build-wscale/frozen_mpy.o] Error 1