Don't error out in system hook if user has `nil` datetime columns

Deleting a user would fail in the system hooks if the user had
`nil` column in `datetime` or `updated_at` fields.

Closes #43871

app/services/system_hooks_service.rb

@@ -20,8 +20,8 @@ class SystemHooksService
   def build_event_data(model, event)
     data = {
       event_name: build_event_name(model, event),
-      created_at: model.created_at.xmlschema,
-      updated_at: model.updated_at.xmlschema
+      created_at: model.created_at&.xmlschema,
+      updated_at: model.updated_at&.xmlschema
     }
 
     case model

changelogs/unreleased/sh-fix-issue-43871-system-hooks.yml

+---
+title: Don't error out in system hook if user has `nil` datetime columns
+merge_request:
+author:
+type: fixed

spec/services/system_hooks_service_spec.rb

@@ -70,6 +70,14 @@ describe SystemHooksService do
       expect(data[:project_visibility]).to eq('private')
     end
 
+    it 'handles nil datetime columns' do
+      user.update_attributes(created_at: nil, updated_at: nil)
+      data = event_data(user, :destroy)
+
+      expect(data[:created_at]).to be(nil)
+      expect(data[:updated_at]).to be(nil)
+    end
+
     context 'group_rename' do
       it 'contains old and new path' do
         allow(group).to receive(:path_was).and_return('old-path')