FIX: Require q param in /tags/filter/search route (#8263)

* Require q param in /tags/filter/search route.
* If not provided this route was causing a 500 error when
  DiscourseTagging.clean_tag was called, because .downcase
  was being called on the param (which was nil).
* Now return a 400 error instead.

app/controllers/tags_controller.rb

@@ -193,6 +193,8 @@ class TagsController < ::ApplicationController
   end
 
   def search
+    params.require(:q)
+
     clean_name = DiscourseTagging.clean_tag(params[:q])
     category = params[:categoryId] ? Category.find_by_id(params[:categoryId]) : nil