Allow empty submodules property in Depends item.

It just means that no dependency should be pulled in by this item.
Useful for this kind of thing:
Product {
    Depends {
        name: "Qt"
        submodules: theQtSubModulesINeed
    }
    property stringList theQtSubModulesIneed // Can be empty.
}

Change-Id: I8e500c3acfa4374be9005fcf349c0417ad47a76e
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>