Allow empty platform path when setting build env for Apple platforms.
Allow empty platform path when setting build env for Apple platforms.

Task-number: QBS-344

Change-Id: If97f45efdeeedbea2e3b21e40bdb9092b605e742
Reviewed-by: Ivan Komissarov <96f14e4d07227e958f51f7d2898113639e9498d9@me.com>
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

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