[Bf-python] normalze zero len vector

[Campbell Barton]

At the moment normalizing a zero length vector makes the vector (nan, 
nan, nan) on linux- which I had checks for in one of my exporters.
however on win32 it became (-1.#INF, -1.#INF, -1.#INF) - which Im not 
sure how to check for.

Could we make it that .normalize() does nothing if the vectors length is 
zero?

In the case of exporters, zero length normals come from faces with no area.

- Cam