external subjects in UI shows "null" for institution

I'm having a problem with the descriptions generated for external users.
In grouper.properties, there is this expression for generating the
description:

> externalSubjects.desc.el = [unverifiedInfo] ${grouperUtil.appendIfNotBlankString(externalSubject.name, ' - ', externalSubject.institution)} [externalUserID] ${externalSubject.identifier}

That expression seems to work fine when the institution was specified on
the registration screen. However, if the institution is left empty on
the form, the term
${grouperUtil.appendIfNotBlankString(externalSubject.name, ' - ',
externalSubject.institution)} doesn't work as intended. Instead of
evaluating to just the subject name, it comes out in the description as
"null". For example, a description like this:

> [unverifiedInfo] null [externalUserID] AItOawn8Oz9QRuePe93O37SkVFluEhOsXSnapuo@google.com is a direct member

I tried a few possibilities for fixing this, but always got "null".

Peter