FIX: Respond to user search correctly when category_id is blank

Previously it would search for category_id=0, which does not exist. With the new permission checks, this returns a 404

app/controllers/users_controller.rb

@@ -921,8 +921,7 @@ class UsersController < ApplicationController
     topic_id = params[:topic_id]
     topic_id = topic_id.to_i if topic_id
 
-    category_id = params[:category_id]
-    category_id = category_id.to_i if category_id
+    category_id = params[:category_id].to_i if category_id.present?
 
     topic_allowed_users = params[:topic_allowed_users] || false
 

spec/requests/users_controller_spec.rb

@@ -3103,6 +3103,15 @@ describe UsersController do
       expect(json["users"].map { |u| u["username"] }).to include(privileged_user.username)
     end
 
+    it "interprets blank category id correctly" do
+      pm_topic = Fabricate(:private_message_post).topic
+      sign_in(pm_topic.user)
+      get "/u/search/users.json", params: {
+        term: "", topic_id: pm_topic.id, category_id: ""
+      }
+      expect(response.status).to eq(200)
+    end
+
     context "when `enable_names` is true" do
       before do
         SiteSetting.enable_names = true