There's a lot of very judgemental people here...
Just because a parking spot has the wheelchair symbol painted on the ground, if a blue sign is not directly in front of the spot, then it is not considered a handicapped spot according to just about every city ordinance I've ever encountered. For all you know, the sign could have been removed due to an upcoming reconfiguring and re-painting of the parking lot.
The OP was asking if it's legal to park in a spot if the sign is not there. And the answer is yes, it is legal.
If there are no parking spots (even if marked on the ground) with an ADA-approved placard posted in front of them, then the land lord is in violation of city ordinance and is liable to receive some hefty fines.
All that being said, if the parking lot is empty, why even bother picking a handicapped spot, whether the sign is there or not?
