allow metadata of tracks without album and tracknumber in database

Summary:
remove not null constrain for database and change sql query to allow tracks with more incomplete metadata

CCBUG: 389136
CCBUG: 393118
CCBUG: 396607

Test Plan: tests pass as before, new tests pass as well

Reviewers: mgallien

Reviewed By: mgallien

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D13685