FIX: Don't raise error in update username job if user has been deleted.

spec/jobs/update_username_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe Jobs::UpdateUsername do
+  fab!(:user) { Fabricate(:user) }
+
+  it 'does not do anything if user_id is invalid' do
+    events = DiscourseEvent.track_events do
+      described_class.new.execute(
+        user_id: -999,
+        old_username: user.username,
+        new_username: 'somenewusername',
+        avatar_template: user.avatar_template
+      )
+    end
+
+    expect(events).to eq([])
+  end
+end