Add check in logic for user when loading non-published post

Alternative to error page before:

`Cannot read properties of undefined (reading '_id')` during checking `!res.locals.post.author._id.equals(req.user._id)` in line 268.

app/controllers/post.js

@@ -260,7 +260,8 @@ class PostController extends SiteController {
   async getView (req, res, next) {
     const { comment: commentService, resource: resourceService } = this.dtp.services;
     try {
-      if ((res.locals.post.status !== 'published') &&
+      if ((res.locals.post.status !== 'published') && 
+          req.user &&
           !res.locals.post.author._id.equals(req.user._id) &&
           !req.user.hasAuthorDashboard) {
         throw new SiteError(403, 'The post is not published');