The following test triggers UB on master, while using public-facing APIs:

#[test]
fn write_null_input_stream() {
    let mut machine = MachineBuilder::new()
        .with_streams(StreamConfig::in_memory())
        .build();

    let results = machine
        .run_query("current_input(Stream), write(Stream, hello).")
        .collect::<Vec<_>>();

    assert_eq!(results.len(), 1);
    assert!(results[0].is_err());
}

This PR fixes #2801, by:

• Ensuring that null streams aren't returned as null Cons (they instead return atom!("null_stream"), which is mapped to Stream::Null(Default::default()) in Machine::configure_streams().
• Turning the stream_as_cell! macro into an impl From<Stream> for HeapCellValue.
• Making it so that null streams can be written to (voiding the stuff written) and read from (always returning eof).

I left in the sanity checks used while debugging the issue, in case someone inadvertently creates another null Cons in the future.