The Email address is not valid. although a default should be used

Problem/Motivation

When adding a User registration notification settings

1. "Send Email to custom Email address(es)"
2. Leave "Email address(es) to notify" empty
3. Click save
4. See error "The Email address is not valid."

This should not happen because the email field does not need to be filled out according to the help text: "Leave empty to use the default system Email address"

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes