Make matutils.unitvec always return float norm when requested

[Witiko]

Contrary to the documentation, matutils.unitvec(…, return_norm=True) function does not always return a float norm:

1. Zero vectors stored as numpy arrays will return an integer norm.
2. Zero vectors stored as lists of tuples (the Gensim BOW format) will return no norm.

This PR applies minimum changes to fix these two issues.

[piskvorky]

Nice catch! Thanks for the tests.