Fixes some weird travis failures noted on master and in #236

On [this line](https://github.com/rspec/rspec-expectations/blob/master/lib/rspec/expectations/differ.rb#L20) we diff data_old with some other things. Diff-lcs does that[0].encoding which it can't get if it's empty. This pull takes an empty list and puts a single empty string in. This passes the specs but is ugly.