chore(lib): add assertion for script to not be null when type=="script"

lib/default.nix

@@ -24,7 +24,8 @@
       if format == "json"
       then expected
       else lib.generators.toPretty {} expected;
-  in {
+  in
+    assert lib.assertMsg (!(type == "script" && script == null)) "test ${name} has type 'script' but no script was passed"; {
       inherit type name description;
       actual = actual';
       expected = expected';