executor: fix check for generated artifacts
executor: fix check for generated artifacts

Instead of skipping artifacts without a transformer we rather check
whether the artifact is of type Generated.
Every generated artifact has a transformer and every non-generated
artifact must not have one.

Change-Id: I1c58b6d32d79c4df4b01a5f8996b28fff4735ab4
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@digia.com>

qtproject/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,Distrotech/qbs,gatzka/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,villytiger/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,villytiger/qbs,Distrotech/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,Distrotech/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qt-labs/qbs,villytiger/qbs,villytiger/qbs