Bus interactions with no witness columns fail

[georgwiese]

Consider this ASM file:

// we don't need to specify an operation_id if we have a single operation
machine SingleOperation with
    degree: 8,
    latch: latch
{
    operation nothing;

    col fixed latch = [1]*;
    col witness w;
    w = w * w;
}

machine Main with degree: 8 {
    SingleOperation m;

    link => m.nothing();

    col witness w;
    w = w * w;
}

This currently fails:

$ cargo run pil test_data/asm/single_operation.asm -o output -f --linker-mode bus --prove-with mock
...
thread 'main' panicked at /Users/georg/coding/powdr/backend-utils/src/lib.rs:218:26:
Identity references no namespace: Constr::PhantomBusInteraction(1, [0]);

The issue is that we can't associate an identity to a PIL namespace if it doesn't reference a column.

I wouldn't expect this to happen in practice, but you could think of cases where this would happen: If you have a stateful machine (e.g. one that interacts with memory) and no arguments. But actually, if it interacts with memory, it would at least receive the current time step, so I'm not sure if this is worth fixing...