Allow `null` values in objects

I'm not sure if stripping null is intended in the first place, but I
feel it's pretty weird. We're using propex to validate our
incoming POST data, so the previous behaviour is causing all "nulls" to
be dropped.

Edit: I'm really not sure about my fix, I don't fully grasp all the edge cases propex tries to handle. Feel free to point me in the right direction if you think this isn't a proper fix.