Inconsistent error message for null/nonnull empty dynarrays.

[skejeton]

type Control = struct {
    things: []int
}

fn main() {
    var x: Control
    // comment and uncomment the line below
    // x.things = {}
    printf("%v", x.things[0])
}

Commented:
Runtime error /playground.um (7): Dynamic array is null
Uncommented:
Runtime error /playground.um (8): Index 0 is out of range 0...-1