When a layer has an empty feature, often due to clipping, it causes MarshalGzipped (and probably Marshal too) to fail with a null pointer panic. The panic is on accessing f.Geometry in the marshaller. On further investigation, this happens when the geometry or feature is nil.

My workflow looks a bit like this:

// get tileData from source
layers, err := mvt.UnmarshalGzipped(tileData)
if err != nil {
	return nil, err
}
layers.ProjectToWGS84(tileInfo)
layers.Clip(requestedTile.Bound())
layers.ProjectToTile(requestedTile)
data, err := mvt.MarshalGzipped(layers)

mvt.MarshalGzipped(layers) fails with a panic.

If I add a Prune function, something along the lines of:

func (l *Layer) Prune() {
	feats := []*geojson.Feature{}
	for _, f := range l.Features {
		if f != nil && f.Geometry != nil {
			feats = append(feats, f)
		}
	}
	l.Features = feats
}

then the tiles marshal and render correctly.

Can I suggest or open a PR to provide a Prune function for both mvt.Layer and mvt.Layers?