One cannot use an empty list to override file tags

For instance:

CppApplication {
    Group {
        name: "Templates (not to be compiled")
        files: "template.cpp"
        fileTags: []
    }
}

This will try to compile template.cpp, because apparently the evaluation code in qbs does not differentiate between "property not set" and "property set to an empty list". I think we can do better.