FIX: Linking to `/new-topic` from `/latest`

`/new-topic` redirects back to `/latest`, so the ember router considers this to be a 'refresh'. This triggers the `resetParams()` logic, which would cause the transition to abort, and the composer would never open.

This commit fixes the `resetParams()` logic so that it uses the default values, instead of setting everything to `null`. Therefore the transition is no longer aborted, and the composer opens successfully.

Unfortunately I have not been able to reproduce the issue in a QUnit test.

app/assets/javascripts/discourse/controllers/discovery/topics.js.es6

@@ -25,9 +25,10 @@ const controllerOpts = {
   expandAllPinned: false,
 
   resetParams() {
-    Object.keys(this.get("model.params") || {}).forEach(key =>
-      this.set(key, null)
-    );
+    Object.keys(this.get("model.params") || {}).forEach(key => {
+      // controllerOpts contains the default values for parameters, so use them. They might be null.
+      this.set(key, controllerOpts[key]);
+    });
   },
 
   actions: {