SceneDataTransformer promotes annotations: undefined to []

[gtk-grafana]

What happens

SceneDataTransformer.transform() always returns an array for annotations, even when the source had none:

// packages/scenes/src/querying/SceneDataTransformer.ts:348
return { ...data, series, annotations };

annotations is initialised to [] and only receives frames tagged
meta.dataTopic === DataTopic.Annotations, so a panel with no annotations goes from
annotations: undefined (what SceneQueryRunner emits _combineDataLayers only assigns when
_layerAnnotations.length > 0) to annotations: [].

Why it matters

Consumers guard on presence, not length. In Grafana:

• TimeSeriesPanel.tsx: {data.annotations && <ExemplarsPlugin … />}
• CandlestickPanel.tsx: same guard

Both now mount ExemplarsPlugin on every panel that has any transformation, with nothing to draw.
ExemplarsPlugin renders EventsCanvas, which registers a uPlot draw hook that bumps React
state on every draw — so pan, zoom and cursor redraws each cost an extra render pass per panel.

This affects any panel with user transformations today.

Suggested fix

annotations: annotations.length > 0 || data.annotations !== undefined ? annotations : undefined,

The data.annotations !== undefined clause is needed: the shorter
annotations.length ? annotations : data.annotations resurrects annotations that an
annotations-topic transformation deliberately removed. This form keeps "transformed to nothing" as
[] and returns undefined only for "never had any".

transform() is the only promotion site: SceneQueryRunner._combineDataLayers already guards.