Do not lose rescuable artifact data after an incomplete build.

Task-number: QBS-630
Change-Id: Ib69c541a6c1e228b1ad3c06da51d905194ed10d8
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Correctly handle overriding of file tags via an empty list.

Task-number: QBS-633
Change-Id: I4bf750ff94d6fae38fb3fdb49de265caddcd789e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>