It does not make sense for a DMA engine to transport `void` data since there's nothing to transport aside from 'valid' counts. Move the `i1` hack into the DMA engine instantiation site to reflect the same decision on the runtime side. Since zero bits is an edge case, this means that DMA engines don't have to reason about it.

Also fix some related bugs.

Assisted-by: vscode:Claude Opus 4.6