We were showing the header when there were no search terms but there
were advanced search filters, unlike what we do for debates and
proposals. Besides, we were already hiding the header when there were
search terms, so it makes sense to hide it when using the advanced
search too.
We're using the `@search_terms` and `@advanced_search_terms` instance
variables in order to be consistent with what we do in the debates and
proposals sections.