Fix USE_BACKEND (source-academy#516)

* Bump @types/node from 9.6.28 to 11.13.0 (source-academy#513)

Bumps [@types/node](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/HEAD/types/node) from 9.6.28 to 11.13.0.
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits/HEAD/types/node)

Signed-off-by: dependabot[bot] <[email protected]>

* fix use backend

Author: jiachen247

src/utils/constants.ts

@@ -6,8 +6,7 @@ export const IVLE_KEY = process.env.REACT_APP_IVLE_KEY;
 export const VERSION = process.env.REACT_APP_VERSION;
 export const BACKEND_URL = process.env.REACT_APP_BACKEND_URL;
 export const USE_BACKEND =
-  process.env.REACT_APP_USE_BACKEND !== '' &&
-  process.env.REACT_APP_USE_BACKEND!.toUpperCase() === 'TRUE';
+  process.env.REACT_APP_USE_BACKEND && process.env.REACT_APP_USE_BACKEND.toUpperCase() === 'TRUE';
 
 export enum LINKS {
   GITHUB_ISSUES = 'https://github.com/source-academy/cadet-frontend/issues',