[Bug] Fix user role from removing email

[esizer]

🤖 Resolves #6103

👋 Introduction

This prevents role updates from overriding the user email to null.

🕵️ Details

We were setting the users email to null when it did not exist (either undefined or empty string) to prevent our validation from seeing "" as a duplicate email. This unsets the email key in our mutation variables if it is undefined so that it is only nulled when it is an empty string.

🧪 Testing

Assist reviewers with steps they can take to test that the PR does what it says it does.

1. Build the application npm run build
2. Navigate to `/admin/users/{userId}/edit
3. Add a role, confirm email is still present
4. Remove a role, confirm email is still present
5. Empty out the email, confirm it saves as empty
6. Attempt to update to an existing email, confirm it fails with nice error message
7. Attempt to update to a unique email, confirm it succeeds

[petertgiles]

Perfection! Nice simple solution. 🎯