ENH: Fix normalization for null vectors and return the norm

First, make sure that null vector (vectors initialized with 0.0) stay
null when normalized.
Also, return the norm used in the normalization as to simplify the checks
when coding with a vector.

ITK-3306

Change-Id: I8103828e9e78e3e627a22fa7a68bd8dda1f764c3
Merge topic 'ImproveITKVector'

088bcb57 ENH: Fix normalization for null vectors and return the norm