test: allow to set NULL to intro or outro

Addresses https://github.com/systemd/systemd/pull/22338#discussion_r796741033.

src/test/test-unit-serialize.c

@@ -60,4 +60,4 @@ static int intro(void) {
         return EXIT_SUCCESS;
 }
 
-DEFINE_CUSTOM_TEST_MAIN(LOG_DEBUG, intro, test_nop);
+DEFINE_TEST_MAIN_WITH_INTRO(LOG_DEBUG, intro);