Installing a stud wall without connecting to ceiling?
Often walls in office buildings do not extend to the roof or floor above. (Suspended ceilings are installed in the offices.) 

If the room is about 12’ or so with a double top plate, it probably doesn’t need to be braced. If it’s over 12’ or so, I’d add a 2x4 brace from the top of the wall up to the floor above.