To keep in line with ruby semantics of [`String#slice`](http://ruby-doc.org/core-2.0/String.html#method-i-slice) slice should return an empty string instead of nil when the starting index is valid even if the length is 0. Fixes #1591
To keep in line with ruby semantics of [`String#slice`](http://ruby-doc.org/core-2.0/String.html#method-i-slice) slice should return an empty string instead of nil when the starting index is valid even if the length is 0. Fixes #1591