test: add undefined fatalException exit code test

Add a test that checks the exit code when _fatalException
is undefined

PR-URL: #38119
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/fixtures/process-exit-code-cases.js

@@ -122,6 +122,15 @@ function getTestCases(isWorker = false) {
     result: isWorker ? 1 : 7,
     error: /^Error: ok$/,
   });
+
+  function exitWithUndefinedFatalException() {
+    process._fatalException = undefined;
+    throw new Error('ok');
+  }
+  cases.push({
+    func: exitWithUndefinedFatalException,
+    result: 6,
+  });
   return cases;
 }
 exports.getTestCases = getTestCases;