FIX: current user serializer consistently returns {} for custom_fields

Resolves: #5210

app/serializers/current_user_serializer.rb

@@ -135,7 +135,7 @@ class CurrentUserSerializer < BasicUserSerializer
     end
 
     if fields.present?
-      User.custom_fields_for_ids([object.id], fields)[object.id]
+      User.custom_fields_for_ids([object.id], fields)[object.id] || {}
     else
       {}
     end

app/serializers/user_serializer.rb

@@ -388,7 +388,7 @@ class UserSerializer < BasicUserSerializer
     end
 
     if fields.present?
-      User.custom_fields_for_ids([object.id], fields)[object.id]
+      User.custom_fields_for_ids([object.id], fields)[object.id] || {}
     else
       {}
     end

spec/components/plugin/instance_spec.rb

@@ -149,6 +149,14 @@ describe Plugin::Instance do
   end
 
   context "serialized_current_user_fields" do
+    before do
+      DiscoursePluginRegistry.serialized_current_user_fields << "has_car"
+    end
+
+    after do
+      DiscoursePluginRegistry.serialized_current_user_fields.delete "has_car"
+    end
+
     it "correctly serializes custom user fields" do
       DiscoursePluginRegistry.serialized_current_user_fields << "has_car"
       user = Fabricate(:user)
@@ -157,6 +165,18 @@ describe Plugin::Instance do
 
       payload = JSON.parse(CurrentUserSerializer.new(user, scope: Guardian.new(user)).to_json)
       expect(payload["current_user"]["custom_fields"]["has_car"]).to eq("true")
+
+      payload = JSON.parse(UserSerializer.new(user, scope: Guardian.new(user)).to_json)
+      expect(payload["user"]["custom_fields"]["has_car"]).to eq("true")
+
+      UserCustomField.destroy_all
+      user.reload
+
+      payload = JSON.parse(CurrentUserSerializer.new(user, scope: Guardian.new(user)).to_json)
+      expect(payload["current_user"]["custom_fields"]).to eq({})
+
+      payload = JSON.parse(UserSerializer.new(user, scope: Guardian.new(user)).to_json)
+      expect(payload["user"]["custom_fields"]).to eq({})
     end
   end