fix userAccess error handling, test non-existing user

frontend/frontend/src/api/user.js

@@ -36,7 +36,7 @@ const getUserMetadata = async (userKey) => {
     return result.data;
   } catch (error) {
     console.error(error);
-    return '';
+    return null;
   }
 };
 

frontend/frontend/src/pages/Home/Home.js

@@ -151,6 +151,7 @@ const HomeScreen = () => {
           const ghToken = prompt('input GitHub token:');
           process.env['PAT_1'] = ghToken;
           await testEndpoint('/api/wakatime?username=ffflabs');
+          await testEndpoint('/api/wakatime?username=ffflabsASDF');
           console.log('PAT_1: ', process.env.PAT_1);
           // console.log(router);
           /*