Fix access to null object in MSVC linker rules.
Fix access to null object in MSVC linker rules.

We no longer require object files to be present when linking.

Change-Id: I7110df35e65578964b7f4b5d12b8b10a5854cc01
Reviewed-by: Jake Petroules <2d627e1cecd5435752fa500bacbe05f7cc90c369@petroules.com>
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

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