As you are evaluating a blank field, we recommend enclosing the first tag in quotes as well. It's usually best practice to encompass all evaluations and statements in double quotes. The true statement would also need to be enclosed with double quotes as it appears to be missing in your sample.

Skeleton Structure Sample: IF "" = "" "" ""

e.g. IF "<<Contact_Mailingxyz>>" <> "" "<<Contact_Mailingxyz>>" ""

Here is a link to our documentation in regards to utilizing Conditional Fields in Word:

http://help.nintex.com/en-US/docgen/docservices/#LOOP%20Document%20Services/Conditional%20Text%20in%20Word.htm?TocPath=L…

Thanks very much!