Can the airbag deploy when you brake hard without hitting anything?

Yes, Airbags are activated by accelerometers, not by contacts. If you change speed more than you should, the accelerometer picks this up and tells a computer to set off the airbag, which it does in time to prevent serious injury. This is regardless of the cause of the change in speed.

And no, hitting the steering wheel will not set an airbag off; it neither makes an accelerometer work enough to notice a change, and would be a rather dangerous idea since most car wheels also carry horn and other function controls.