qmlsc miscompiles registers only set in in one of two alternative branches of execution

Analogous to QTBUG-104462, at the beginning of each function we need to initialize locals that are not overwritten in all possible branches. They should initially be undefined/void.

The "inverse" of QTBUG-104665 can be used as an example, or even simpler:

function a() : string {
    var x;
    return x + "";
}

What we want here is "undefined", not the empty string.