[RUN AS-IF-FOSS] Fix partial not existing in FOSS

What does this MR do?

Fixes #229728 (closed)

layouts/mailer/additional_text only exists in ee. So use render_if_exists

$ find . | grep layouts/mailer/_additional_text
./ee/app/views/layouts/mailer/_additional_text.text.erb
./ee/app/views/layouts/mailer/_additional_text.html.haml

Reproduction

# Rename the ee folder first
mv ee ee_old

FOSS_ONLY=1 ENABLE_SPRING=1 bin/rspec spec/services/notification_service_spec.rb