liveview: interesting behaviour in trans mode on ZOOM

liveview behaviour in transmission mode seems to have changed. In this mode there is no actual detector data, but the image seems to show the last sans image but flipping it from side to side! previously it either showed blank or a static last image, not normally causing a problem, but the scripting was failing to change wiring tables to sans so trans flipping an old sans image was confusing. It would be better if it showed blank.

## Acceptance criteria

in transmisison mode, liveview display is blank