[FIX] website_blog: fix the blogs search

Before this commit, following these steps:
- Go to /blog
- Enable the customize option 'Top banner - Name / Latest Post'
- Disable the customize option 'Full-Width Cover'
- Search for the word 'helicopter' in the blog post search bar

Two problems are then visible:
1) The top banner is always visible with a blog post that does not match
the search. This top banner should not be visible (like for tag and date
searches).
2) There is an alert message 'No results for "helicopter".' while the
blog post 'Maui helicopter tours' matches the search.

This commit corrects both errors.

task-3058239

Part-of: odoo/odoo#105462