User Picker - do not allow to leave it empty

[Juan_Jose_Andres_Gerez]

good afternoon.
I do not know if you can help me with a question.

I have a user field (user picker) that I want users to be able to change at any time, but they can not leave it empty.

I do not have to do it with a validation in the Workflow because when the change is made, a webhook is sent to an external application that is dependent on what happens in JIRA and the problem is that when someone by mistake or intentionally deletes the user field, this external application "corrupts", it stops synchronizing correctly.

For the moment, we have left the external application that the previous user keeps in case it arrives empty in the webhook so that it does not fail, but theoretically, that user for security should never be deleted ...

Any ideas on how to do this?
I have been looking at the script runner except to do a validation, or a postfunction in the Workflow I do not see any option. And what I would like is that it be done as a kind of validation "live" and do not allow the user to empty.

PS: I also do not want to put the user as mandatory, because when the issue is created, that user does not yet exist in the process.

[Nic Brough -Adaptavist-]

Set the field as mandatory in the field configuration.  Your users will not be able to leave it empty at any time then.

[Juan_Jose_Andres_Gerez]

Thank you very much for your contribution.
But as I said, that option does not work for me because it would force me to introduce the user from the moment of creation, and this is not the case, since this user is filled in the third step of the Workflow and from that moment is when you can change , but do not leave empty.