handle an edge case in zgradient where z is not set

[kylegrover]

I was running into this when a sketch started with extruder off, then extruder on, before a point. resulting in an error. this should safely handle that case

[AndyGlx]

I agree that this should be handled, but i think this would also be an issue for non-Z values, especially if another colour-type was chosen. I think a better solution is for the visualization function to identify if a path (points between extruder being turned off and on or vice versa contains no points, or only one point. I thought there was some kind of check for this.