Hi Paul,
Try the following expression, that should work:
pr_header.date_deliver.isoValue.trim().isEmpty()?"<replacement_string>":Date.fromIsoValue(pr_header.date_deliver.isoValue).format("DD MMM YYYY"),
where <replacement_string> is a string of your choice (can be empty).
But normally, since version 2.50, you should be able to test the value of the variable with the 'isNull()' RTL function, but it doesn't seem to work (I made several tests on that, but always got the same error); we'll investigate on that.
Best regards,
Romain