Make AbuseReportMailer responsible for knowing if it should deliver

app/mailers/abuse_report_mailer.rb

@@ -2,6 +2,8 @@ class AbuseReportMailer < BaseMailer
   include Gitlab::CurrentSettings
   def notify(abuse_report_id)
+    return unless deliverable?
+
     @abuse_report = AbuseReport.find(abuse_report_id)
     mail(
@@ -9,4 +11,10 @@ class AbuseReportMailer < BaseMailer
       subject:  "#{@abuse_report.user.name} (#{@abuse_report.user.username}) was reported for abuse"
     )
   end
+
+  private
+
+  def deliverable?
+    current_application_settings.admin_notification_email.present?
+  end
 end

spec/mailers/abuse_report_mailer_spec.rb

+require 'rails_helper'
+
+describe AbuseReportMailer do
+  include EmailSpec::Matchers
+
+  describe '.notify' do
+    context 'with admin_notification_email set' do
+      before do
+        stub_application_setting(admin_notification_email: 'admin@example.com')
+      end
+
+      it 'sends to the admin_notification_email' do
+        report = create(:abuse_report)
+
+        mail = described_class.notify(report.id)
+
+        expect(mail).to deliver_to 'admin@example.com'
+      end
+
+      it 'includes the user in the subject' do
+        report = create(:abuse_report)
+
+        mail = described_class.notify(report.id)
+
+        expect(mail).to have_subject "#{report.user.name} (#{report.user.username}) was reported for abuse"
+      end
+    end
+
+    context 'with no admin_notification_email set' do
+      it 'returns early' do
+        stub_application_setting(admin_notification_email: nil)
+
+        expect { described_class.notify(spy).deliver_now }.
+          not_to change { ActionMailer::Base.deliveries.count }
+      end
+    end
+  end
+end