fix debug boards view

uttt.c

@@ -151,7 +151,7 @@ void ut_show(const struct ut_state *state) {
 void ut_show_boards(const struct ut_state *state) {
 	for (int j = 0; j < 3; j++) {
 		for (int i = 0; i < 3; i++) {
-			putchar(state->boards[j][i]);
+			putchar(state->boards[j][i] ? state->boards[j][i] : ' ');
 		}
 		putchar('\n');
 	}