XPath for tag input check

I have a screen with a panel Credential containing a tag input Users pointing to entity CredentialUser and field refBusinessPartnerId. The tag input must not be empty. How do I write the test condition in the confirmation rule so that the record cannot be saved with the tag input empty? Not(string(@Users)) or count or not(string(CredentialUser/@refBusinessPartnerId)) didn’t work.

The array field in the top entity should contain a number of child records. So testing for ‘taginputfield = 0’ might do the job.