Complete zero-QWC terminal DMA chains

[GTTeancum]

Summary

• complete terminal GIF DMA chains even when their END tag has zero QWC
• avoid submitting an empty GIF packet while still clearing CHCR.STR and raising D_STAT
• add a regression test covering the terminal tag state and interrupt

Why

A zero-QWC END tag currently leaves the channel active forever because no pending transfer is queued when the accumulated payload is empty. The terminal tag is still a completed DMA operation and needs the normal completion bookkeeping.

Testing

• 426/426 ps2x_tests passed (Release)