#472 The variable mh-unseen-seq is set when Unseen_Sequence does not exist

[Bill Wohler]

Sergey points out that we set mh-unseen-seq to "unseen" if Unseen-Sequence is not defined. His feeling is that we should leave mh-unseen-seq as nil in this case.

Thus, remove this case and check that the code handles a nil mh-unseen-seq gracefully.