tools/mpy-tool.py: Don't generate const_table if it's empty.

tools/mpy-tool.py

@@ -331,9 +331,11 @@ class RawCode:
                 # TODO
                 raise FreezeError(self, 'freezing of object %r is not implemented' % (obj,))
 
-        # generate constant table
+        # generate constant table, if it has any entries
+        const_table_len = len(self.qstrs) + len(self.objs) + len(self.raw_codes)
+        if const_table_len:
             print('STATIC const mp_rom_obj_t const_table_data_%s[%u] = {'
-            % (self.escaped_name, len(self.qstrs) + len(self.objs) + len(self.raw_codes)))
+                % (self.escaped_name, const_table_len))
             for qst in self.qstrs:
                 print('    MP_ROM_QSTR(%s),' % global_qstrs[qst].qstr_id)
             for i in range(len(self.objs)):
@@ -362,7 +364,10 @@ class RawCode:
         print('    .n_pos_args = %u,' % self.prelude[3])
         print('    .data.u_byte = {')
         print('        .bytecode = bytecode_data_%s,' % self.escaped_name)
+        if const_table_len:
             print('        .const_table = (mp_uint_t*)const_table_data_%s,' % self.escaped_name)
+        else:
+            print('        .const_table = NULL,')
         print('        #if MICROPY_PERSISTENT_CODE_SAVE')
         print('        .bc_len = %u,' % len(self.bytecode))
         print('        .n_obj = %u,' % len(self.objs))