Add check for missing tag param in streaming API (#8955)

* Add check for missing tag param in streaming API

Fixes error:

```
TypeError: Cannot read property 'toLowerCase' of undefined
at app.get (.../streaming/index.js:493:50)
```

* Fix code style issues

streaming/index.js

@@ -449,6 +449,11 @@ const startWorker = (workerId) => {
     });
   };
 
+  const httpNotFound = res => {
+    res.writeHead(404, { 'Content-Type': 'application/json' });
+    res.end(JSON.stringify({ error: 'Not found' }));
+  };
+
   app.use(setRequestId);
   app.use(setRemoteAddress);
   app.use(allowCrossDomain);
@@ -490,11 +495,25 @@ const startWorker = (workerId) => {
   });
 
   app.get('/api/v1/streaming/hashtag', (req, res) => {
-    streamFrom(`timeline:hashtag:${req.query.tag.toLowerCase()}`, req, streamToHttp(req, res), streamHttpEnd(req), true);
+    const { tag } = req.query;
+
+    if (!tag || tag.length === 0) {
+      httpNotFound(res);
+      return;
+    }
+
+    streamFrom(`timeline:hashtag:${tag.toLowerCase()}`, req, streamToHttp(req, res), streamHttpEnd(req), true);
   });
 
   app.get('/api/v1/streaming/hashtag/local', (req, res) => {
-    streamFrom(`timeline:hashtag:${req.query.tag.toLowerCase()}:local`, req, streamToHttp(req, res), streamHttpEnd(req), true);
+    const { tag } = req.query;
+
+    if (!tag || tag.length === 0) {
+      httpNotFound(res);
+      return;
+    }
+
+    streamFrom(`timeline:hashtag:${tag.toLowerCase()}:local`, req, streamToHttp(req, res), streamHttpEnd(req), true);
   });
 
   app.get('/api/v1/streaming/list', (req, res) => {
@@ -502,8 +521,7 @@ const startWorker = (workerId) => {
 
     authorizeListAccess(listId, req, authorized => {
       if (!authorized) {
-        res.writeHead(404, { 'Content-Type': 'application/json' });
-        res.end(JSON.stringify({ error: 'Not found' }));
+        httpNotFound(res);
         return;
       }
 
@@ -553,9 +571,19 @@ const startWorker = (workerId) => {
       streamFrom(channel, req, streamToWs(req, ws), streamWsEnd(req, ws, subscriptionHeartbeat(channel)), true);
       break;
     case 'hashtag':
+      if (!location.query.tag || location.query.tag.length === 0) {
+        ws.close();
+        return;
+      }
+
       streamFrom(`timeline:hashtag:${location.query.tag.toLowerCase()}`, req, streamToWs(req, ws), streamWsEnd(req, ws), true);
       break;
     case 'hashtag:local':
+      if (!location.query.tag || location.query.tag.length === 0) {
+        ws.close();
+        return;
+      }
+
       streamFrom(`timeline:hashtag:${location.query.tag.toLowerCase()}:local`, req, streamToWs(req, ws), streamWsEnd(req, ws), true);
       break;
     case 'list':