Only set User#notification_email with user input

What does this MR do?

Previously the User "notification preferences" form automatically set a value for User#notification_email. This would become the database value when the user submitted the form, whether the user was aware of this or not. That behavior recently became unnecessary, because the User model now assumes the notification_email is the primary email when not set. Also, that behavior was bad practice because the app should not be creating user input values without the user's knowledge. As a simple solution, this change introduces a blank option to the select dropdown, which is used by default. Now the database value will only change if the user changes it.

We are doing the same thing for User#commit_email in !68591 (merged). This is all part of a broader refactoring of User secondary emails.

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Related to https://gitlab.com/groups/gitlab-org/-/epics/6564