MythMusic: Fix smart playlists uses lastplay or date_entered.

(cherry picked from commit 428609b782c5644b2e885d861f19dbb354f344c0)
MythMusic: Fix sql error in SmartPlaylist editor when adding a new playlist.

(cherry picked from commit e5d6a425053e2c6ce747cf8adfd7aaddf4bcb97a)
MythMusic: Fix NULL insert into NOT NULL column in SmartPlaylist editor.

(cherry picked from commit 43b799a5fc57214102d586a01fc755e541e29ccd)
MythMusic: Allow smart playlist to be saved even if it produces no matches. The nature of smart playlists means that it may match tracks in the future.

(cherry picked from commit 2f3f422e507dd5601aa0b0426bc81f59cda020a7)