Excluding blank or null values from report
I would use the Search Builder (Search menu » Search Builder) to select your field with an operator of "Not Empty" (see screenshot below).  It's not clear when you say "Report" if a search is sufficient - if it is, you're done.

If you DO need a report, add these users to a group (possibly a smart group, depending on whether you need this on an ongoing basis).  Then generate your report, filtering by members of the group.