application controller: fix errors in normalize_search.

Fix exceptions in `normalize_search` on e.g. `https://danbooru.donmai.us/users?search=blah`.
Caused when the `search` param is not a hash.

test/functional/application_controller_test.rb

@@ -35,5 +35,10 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
         assert_response 410
       end
     end
+
+    should "normalize search params" do
+      get tags_path, params: { search: { name: "bkub", post_count: "" } }
+      assert_redirected_to tags_path(search: { name: "bkub" })
+    end
   end
 end