LeaderSegment.resolve() should not return empty string

[bertfrees]

LeaderSegment.resolve() and LeaderSegment.peek() currently return the empty string. The exact string that is returns doesn't matter because it is not literally included in the result, however it can influence the translation of the segments around the leader. The simplest and most accurate thing to do is probably to return a single instance of the leader pattern. The length doesn't matter.

[bertfrees]

See mtmse#30