Add Vector3::getNormalizedIfNotZero() and correct Vector3::getNormalizedOrZero()

The name of the function getNormalizedOrZero() in src/core/math/Vector3.h is somewhat misleading. One might naively expect that the returned Vector3 is either normalized or exactly zero. However, if its length is found to be too small, the function actually returns the original Vector3, the components of which must not be exactly zero.

This merge request renames the above function and all its occurrences to getNormalizedIfNotZero(). It additionally adds the function getNormalizedOrZero() that actually returns a zero-Vector3 when the original vector's length is too small.