fix: name a missing database in one error line

scripts/generate_truth.py

@@ -78,8 +78,9 @@ def main(argv: list[str] | None = None) -> None:
     profiles = load_emulator_profiles(args.emulators_dir)
 
     # Load database (optional)
+    # The default may be absent; a --db that was named must exist.
     db: dict | None = None
-    if os.path.exists(args.db):
+    if args.db != DEFAULT_DB_FILE or os.path.exists(args.db):
         db = load_database(args.db)
 
     # Determine platforms