Get the manual here: https://www.sony.com/electronics/support/e-mount-body-nex-3-series/nex-f3/manuals   There is an option in the set-up menu for release without lens in there.   That should solve your problem

Thanks Pieter for the answer, and the sarcasm!! I've spent hours trawling the internet!

Menu --> Settings --> Shooting settings --> Release w/o Lens --> On   Sometimes I wonder why people take the effort to ask a question while a simple Google-search on 'Sony Alpha NEX-F3 manual' will get you the result in less than a minute...