FrameProps raw data toggle centres if no props loaded

[LightArrowsEXE]

Currently, the "Raw data" toggle is centred in the frame props window if there are no props for it to display.

Minimum reproducible sample:

from vstools import core, vs, set_output

set_output(core.std.BlankClip(format=vs.RGBS))

This will still have _Duration props, but those are ignored by default.