py: Add qstr cfg capability; generate QSTR_NULL and QSTR_ from script.

py/qstr.c

@@ -75,11 +75,9 @@ STATIC const qstr_pool_t const_pool = {
     10,                 // set so that the first dynamically allocated pool is twice this size; must be <= the len (just below)
     MP_QSTR_number_of,  // corresponds to number of strings in array just below
     {
-        (const byte*) "\0\0\0\0", // invalid/no qstr has empty data
-        (const byte*) "\x05\x15\0\0", // empty qstr with hash=5381=0x1505
-#define Q(id, str) str,
+#define QDEF(id, str) str,
 #include "genhdr/qstrdefs.generated.h"
-#undef Q
+#undef QDEF
     },
 };