[cmyth] For protocol >= 77, do not send trailing blank parameter with SET_BOOKMARK

This useless parameter was removed in MythTV 0.27, protocol 77.

Reference:
http://code.mythtv.org/trac/ticket/11104
http://code.mythtv.org/trac/changeset/a8722faea68e8f7d2bb12f31bf78d6bc272dbdde/mythtv

* resolves #139