Only reassign centers if to_reassign.sum() > 1

Sometimes to_reassign has only False values, if that's the case the function
fails. This patch makes it so no reassignment is done in that case.

The patch seems to work here, but maybe to_reassign shouldn't ever be only
False for some reason and there's a bug elsewhere. I hope a more knowledgeble
person can look this over.
TST: add a test for empty reassignment in MBKmeans