Release testing instructions for WC Payments 4.2.0

Fix empty phone and email value

Stripe does not accept empty strings for support phone and email. We don't have to send empty values for these fields.

Testing instructions

• go to Card readers -> Receipt details section
• set an empty value for email -> an error should be displayed and the save button disabled
• set an empty value for phone number -> an error should be displayed and the save button disabled