Allow saving of edited service accounts without email defined

What does this MR do and why?

Before, when the email address was not provided it would sent "null" in the network request. The backend expects the field not be provided at all if we expect the email to be set to a dummy email address in the backend.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

Make sure you have a Ultimate license

Enable the feature flag

• Enable the :edit_service_account_email feature flag in rails console

Feature.enable(:edit_service_account_email)

• As an instance admin navigate to <instance_url>/admin/application_settings/service_accounts

OR

follow the navigation steps as listed in !200406 (comment 2681257283)

• Create a service account
• Attempt to edit a service account by changing either username or name. Leave email blank.

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