autofix: fix ReplaceTrailingDotsWithEllipsis to accept empty source strings

Having the ReplaceTrailingDotsWithEllipsis autofix enabled results in an _IndexError: string index out of range_ error whenever an empty source string is used (which is unlikely, but not technically impossible).

This patch solves this problem by first checking for non-emptiness of the source string.
Don't use len() for string emptiness check
Merge pull request #321 from bertm/patch-1

autofix: fix ReplaceTrailingDotsWithEllipsis to accept empty source strings