TRAP Caching: Unset missing cache rather than setting to undefined

lib/trap-caching.js

@@ -105,7 +105,7 @@ async function downloadTrapCaches(codeql, languages, logger) {
             // still just an empty directory. There's no reason to tell the extractor to use it,
             // so let's unset the entry in the map so we don't set any extractor options.
             logger.info(`No TRAP cache found in Actions cache for ${language}`);
-            result[language] = undefined;
+            delete result[language];
         }
     }
     return result;