Fix nested blank search params not being stripped from searches.

From https://danbooru.donmai.us/forum_topics/9127?page=258#forum_post_151308:

    When you do a user search (from https://danbooru.donmai.us/users/search)
    the results only include those with someone in the inviter field.

The bug was that nested blank search params (/users?search[inviter][name_matches]=)
didn't get stripped from the search.

test/functional/users_controller_test.rb

@@ -26,6 +26,11 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         get users_path, params: {:search => {:name_matches => @user.name}}
         assert_response :success
       end
+
+      should "list all users (with blank search parameters)" do
+        get users_path, params: { search: { inviter: { name_matches: "" }, level: "", name: "test" } }
+        assert_redirected_to users_path(search: { name: "test" })
+      end
     end
 
     context "show action" do