LongestCommonSubstring Can Return null


I ran into a minor issue with LongestCommonSubstring. If either the source or target is empty, the function will return a null which will cause an exception in ApproximatelyEquals.cs when an attempt is made to get the length of the returned value from LongestCommonSubstring. This is happening because LongestCommonSubstring is set to return null if either source or target is null or empty. This is inconsistent behavior as no other function returns null if either source or target is an empty string. For my purposes, I fixed the problem by having the function return String.Empty if either source or target is empty.