Regression: binding to undefined properties yields an error even if the context is disabled with the condition property

Prior to https://codereview.qt-project.org/#change,68396, the following were valid:

Product {
    condition: qbs.targetOS.contains("darwin")
    cpp.infoPlistFile: "Info.plist"
}

Properties {
    condition: qbs.targetOS.contains("darwin")
    cpp.infoPlistFile: "Info.plist"
}

Now, this fails on Windows because the infoPlistFile property is only defined in the Darwin implementation of the cpp module. Surely this should only be an error if the context is enabled.