py/qstr: Rename MP_QSTR_NULL to MP_QSTRnull to avoid intern collisions.

[nevercast]

Fixes #5140

Use MP_QSTRnull instead of MP_QSTR_NULL in much the same way as MP_QSTRnumber_of was used, to avoid collisions.

This allows interning the string "NULL" without it clashing with the special QSTR. (Such as when freezing python code that contains the string "NULL").

[dpgeorge]

Nice fix, exactly what I was thinking to do!

[nevercast]

I'm not sure if it is necessary but I was thinking a frozen "NULL" string inside unix/coverage would have been nice to ensure this doesn't regress in the future. Though I doubt it would. Any thoughts on that?

( I have tested locally that using "NULL" in frozen modules for unix, stm32 and esp32 all pass. Its just not in this PR )

[dpgeorge]

I'm not sure if it is necessary but I was thinking a frozen "NULL" string inside unix/coverage would have been nice to ensure this doesn't regress in the future.

Sounds good, would act as a reminder about this subtle point.

[nevercast]

I can squash this if you like, left it as a seperate commit so that change was obvious.

[dpgeorge]

I think it makes sense to rename the new frozen test file, so it could be used in the future for other qstr-related tests (eg call it frzqstr.py).

[dpgeorge]

Thanks very much, merged in 7d58a19