If you efiled the Federal extension, there must be a FEIN.

If the IRS issued a FEIN they generally state the necessary filing requirement in the letter showing that FEIN. They are most likely gonna' be looking for a F 1065. 

And when/if they show up & ask for it, it may not necessarily politely.  IF there's no income, you'd be able to then explain it to them and have any of the (possibly) proposed penalties abated. 

IMHO, why take the chance of your client getting a nasty-gram & perhaps being upset with you when you could head this off by just filing the return?  Especially since you've already incurred the REP fee (if you aren't on unlimited).