tools/makemanifest.py: Generate working code for empty manifests.

When no usable manifest directives are found (as opposed to no manifest
being set in the makefile), non-compiling code was generated for the
empty frozen constants pool block.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

tools/makemanifest.py

@@ -245,7 +245,7 @@ def main():
             b'#include "py/emitglue.h"\n'
             b"extern const qstr_pool_t mp_qstr_const_pool;\n"
             b"const qstr_pool_t mp_qstr_frozen_const_pool = {\n"
-            b"    (qstr_pool_t*)&mp_qstr_const_pool, MP_QSTRnumber_of, 0, 0\n"
+            b"    (qstr_pool_t*)&mp_qstr_const_pool, MP_QSTRnumber_of, 0, 0, NULL, NULL\n"
             b"};\n"
             b'const char mp_frozen_names[] = { MP_FROZEN_STR_NAMES "\\0"};\n'
             b"const mp_raw_code_t *const mp_frozen_mpy_content[] = {NULL};\n"