py/mpprint: Fix int formatting so "+" is printed for 0-valued integer.

py/mpprint.c

@@ -222,7 +222,7 @@ int mp_print_mp_int(const mp_print_t *print, mp_obj_t x, int base, int base_char
     char prefix_buf[4];
     char *prefix = prefix_buf;
 
-    if (mp_obj_int_sign(x) > 0) {
+    if (mp_obj_int_sign(x) >= 0) {
         if (flags & PF_FLAG_SHOW_SIGN) {
             *prefix++ = '+';
         } else if (flags & PF_FLAG_SPACE_SIGN) {