fix: use child signal for radius if faceted

[ChiaLingWeng]

close #8754
I found that the range for radius using width and height without checking if the model is faceted

vega-lite/src/compile/scale/range.ts

Lines 311 to 314 in c33f46b

	new SignalRefWrapper(() => {
	const w = model.getSignalName('width');
	const h = model.getSignalName('height');
	return `min(${w},${h})/2`;