Don't scrape if no username/password

Instead of having the frontend rely on the scraper giving a success
(a login failure would redirect the user to the login page)
and a blank (default) tableData in the case of a blank username or
password, the backend now detects this and automatically gives
a default response.

serve.js

@@ -196,8 +196,23 @@ app.post('/data', async (req, res) => {
         //
         // Get data from scraper:
         //
-        response = await scraper.scrape_student(req.session.username, req.session.password, req.body.quarter);
-        res.send(response);
+        if (!req.session.username || !req.session.password) {
+            res.send({
+                classes: [],
+                recent: {
+                    recentActivityArray: [],
+                    recentAttendanceArray: []
+                },
+                overview: [],
+                username: "",
+                quarter: "0"
+            });
+        }
+        else {
+            res.send(scraper.scrape_student(
+                req.session.username, req.session.password, req.body.quarter
+            ));
+        }
 
         // If "out" command-line argument provided, save JSON at the given path
         if (args.hasOwnProperty("out")) {