Update conditional when rendering search tabs

** Why are these changes being introduced:

Currently, if the initial DOM rendering is done in a condition where
there are no search results, the UI doesn't not include tabs to change
to a different set of results - which prevents the user from potentially
finding results that were initially excluded.

** Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/use-172

** How does this address that need:

This updates the conditional to look for the presence of a query term,
rather than the presence of results. This allows us to continue to hide
the tabs on initial rendering, prior to a search being done, but should
mean that the tabs are shown in all conditions after a search.

** Document any side effects to this change:

There shouldn't be - the USE application only has a single search
field, so looking for params[:q] should be sufficient to detect a
search. GeoData uses other fields, but it relies on different partials
and doesn't use tabbed results anyway.

Ultimately I could see a helper method for "is there a search?", which
we get close to with the existing readable_param method, but I don't
want to poke this bear until we have a better sense of what a general
solution should look like.

Author: matt-bernhardt

app/views/search/_form.html.erb

@@ -31,6 +31,6 @@
     </aside>
 <% end %>
 
-<%= render partial: 'search/source_tabs' if @results.present? %>
+<%= render partial: 'search/source_tabs' if params[:q].present? %>
 
 <%= javascript_include_tag "search_form" %>