IF/Then to replace a blank parameter in title block format

Is there a way to use a different parameter if your primary parameter is blank that your title block references? I want my REV cell using &ptc_wm_revision to use the parameter drawing_rev if ptc_wm_revision is blank. 

example: if ptc_wm_revision is blank use drawing_rev, but if ptc_wm_revision isn't blank then use ptc_wm_revision.

This would be used if a team is creating drawings outside of windchill due to location, but when we put them into windchill at a later date then the revision assigned from windchill will override the prototype revision.