Add a WARNING log when trying to send an email to a user without any valid email registered

[lemeurherve]

Problem with this page

N/A

Expected behavior

When the app try to send an email to a user, log a WARNING about the email if missing or not valid (ex: the accounts concerned by the LDAP incident from previous years where some emails have been replaced)

Ref: jenkins-infra/helpdesk#3695 (comment)

Actual behavior

No log is emitted.

Possible solution

Add a new item in blockReasons if the user email is empty or not valid: https://github.com/jenkins-infra/account-app/blob/main/src/main/java/org/jenkinsci/account/Application.java#L158-L219