tools: Write msvc-compatible frozen content.

The msvc compiler doesn't accept zero-sized arrays so let the freezing
process generate compatible C code in case no modules are found and the
involved arrays are all empty.  This doesn't affect the functionality in
any way because those arrays only get accessed when mp_frozen_mpy_names
contains names, i.e.  when modules are actually found.

tools/makemanifest.py

@@ -327,7 +327,7 @@ def main():
             b"    (qstr_pool_t*)&mp_qstr_const_pool, MP_QSTRnumber_of, 0, 0\n"
             b"};\n"
             b'const char mp_frozen_mpy_names[1] = {"\\0"};\n'
-            b"const mp_raw_code_t *const mp_frozen_mpy_content[0] = {};\n"
+            b"const mp_raw_code_t *const mp_frozen_mpy_content[1] = {NULL};\n"
         )
 
     # Generate output