Fix for InitializeLegacyUrlReplaceCharacters

When using IsNullOrWhiteSpace the white space rule that will replace white spaces with dashes are ignored. This leads to unexpected behavior. Changed the code to use IsNullOrEmpty instead to match the behavior of the LegacyShortStringHelper.
Merge pull request #484 from sniffdk/patch-1

U4-5527: DefaultShortStringHelper ignores white space rule