addSource geojson with "null" data renders previous geojson

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**mapbox-gl-js version**: 3.7.0

**browser**: Chrome for Linux: Version 129.0.6668.100 (Official Build) (64-bit)

### Steps to Trigger Behavior

1. Add valid mapbox access token to jsfiddle
2. See the orange polygon which gets initialized with geojson data and is rendered properly. Script will remove and add the source + layer every couple of seconds with geojson data `null`:

https://github.com/user-attachments/assets/3ef19542-3094-4944-8aa8-fdf77701742c

Adding a empty featureCollection instead of `null` is behaving as expected:

```
map.addSource('my-source', {
type: 'geojson',
data: data ?? {
type: 'FeatureCollection',
features: []
}
});
```

https://github.com/user-attachments/assets/418554cc-08b1-4ad7-bf28-149503e2f5c5

Should passing `null` as `data` work?

### Link to Demonstration
https://jsfiddle.net/ogy2xwsc/

### Expected Behavior
It renders nothing, since passed geojson data is `null`.

### Actual Behavior
It renders previous existing geojson.

Start with the linked JSFiddle and the map.addSource entry point, reproducing the cycle that removes and re-adds a GeoJSON source with null data. Trace the source update and rendering behavior to determine why earlier GeoJSON remains visible; done means null renders no previous geometry while the empty FeatureCollection behavior remains unchanged.

Written by the indexing model from the issue text.