Whitelist **empty** for skybox

[wrefgtzweve]

Currently flying a helicopter into the skybox can still result in the rotor breaking off, also makes it return early so damage isn't applied twice.

This for example would trigger the bug

An example trace of the case above

["AllSolid"]	=	false
["Contents"]	=	1
["DispFlags"]	=	0
["Entity"]	=	Entity [0][worldspawn]
["Fraction"]	=	1
["FractionLeftSolid"]	=	0.32773450016975
["Hit"]	=	true
["HitBox"]	=	0
["HitGroup"]	=	0
["HitNoDraw"]	=	false
["HitNonWorld"]	=	false
["HitNormal"]	=	0.000000 0.000000 0.000000
["HitPos"]	=	-5493.462891 5308.244629 2849.267822
["HitSky"]	=	false
["HitTexture"]	=	**empty**
["HitWorld"]	=	true
["MatType"]	=	67
["Normal"]	=	-0.784633 0.404783 -0.469576
["PhysicsBone"]	=	0
["StartPos"]	=	-5322.031250 5219.805176 2951.863770
["StartSolid"]	=	true
["SurfaceFlags"]	=	0
["SurfaceProps"]	=	0

I think what's happening is that the trace starts in the world so it hits a inside of a brush.