RADIUS::avp ip4

I have an iRule that is doing persistent records based off of RADIUS AVP pairs. I want to modify it so that it can check to see if the RADIUS::avp 4 ip4 is empty or not.

Currently it is set to: set nas_ip [RADIUS::avp 4 ip4] persist uie $nas_ip 28800

I want to do a check to see if that RADIUS::avp 4 is empty or not. Since it's an ip4 format, I can NOT do a string compare like so: if {[RADIUS::avp 4 ip4] ne ""}

What could I use to see if that AVP has a value in it or not?