Charts: Center funnel empty state via parent Stack alignment

The funnel empty state previously used `padding: 48px 24px` to push
its text away from the container edges, but it sat at the top of the
chart's fixed-height column rather than being vertically centered.
The 48px vertical padding was also the only spacing literal in the
package with no matching WPDS dimension token.

Add `align="center" justify="center"` to the parent Stack in the
empty-state branch so the message is centered both axes within the
chart's resolved height, matching the centering pattern used by
SvgEmptyState. Drop the now-unused padding from .empty-state.

Verified centering against the EmptyData story: chart bounds
(999.5, 492.75) and empty-state bounds match exactly on both axes.

Author: adamwoodnz

projects/js-packages/charts/src/charts/conversion-funnel-chart/conversion-funnel-chart.module.scss

@@ -124,7 +124,6 @@
 
 .empty-state {
 	text-align: center;
-	padding: 48px var(--wpds-dimension-padding-2xl, 24px);
 	color: #6b7280;
 	font-size: var(--wpds-font-size-lg, 16px);
 }

projects/js-packages/charts/src/charts/conversion-funnel-chart/conversion-funnel-chart.tsx

@@ -300,6 +300,8 @@ const ConversionFunnelChartInternal: FC< ConversionFunnelChartProps > = ( {
 		return (
 			<Stack
 				direction="column"
+				align="center"
+				justify="center"
 				data-testid="conversion-funnel-chart"
 				className={ clsx(
 					styles[ 'conversion-funnel-chart' ],