all: Use NULL instead of "" when calling mp_raise exception helpers.

This is the established way of doing it and reduces code size by a little
bit.

extmod/modwebrepl.c

@@ -308,7 +308,7 @@ STATIC mp_obj_t webrepl_set_password(mp_obj_t passwd_in) {
     size_t len;
     const char *passwd = mp_obj_str_get_data(passwd_in, &len);
     if (len > sizeof(webrepl_passwd) - 1) {
-        mp_raise_ValueError("");
+        mp_raise_ValueError(NULL);
     }
     strcpy(webrepl_passwd, passwd);
     return mp_const_none;