Fix promoteId crash on features with null properties

[spokodev]

promoteId throws and crashes the whole geojsonvt(...) call when a feature has properties: null, which is valid GeoJSON (RFC 7946 section 3.2 allows a null properties member).

geojsonvt({
  type: 'Feature',
  properties: null,
  geometry: {type: 'Point', coordinates: [0, 0]}
}, {promoteId: 'id'});
// TypeError: Cannot read properties of null (reading 'id')

A single such feature anywhere in the input produces no tiles. The other id paths already tolerate null properties: generateId: true works, and no id option works, so this is an inconsistency in the promoteId path.

Cause

src/convert.js reads geojson.properties[options.promoteId] with no guard against a null (or absent) properties.

Fix

Guard the access. null && x / undefined && x resolve to a falsy id, which is normalized to no id downstream (the same result as a missing property key), so behavior is unchanged for features that do have properties.

Testing

Added a test that tiles a properties: null feature with promoteId. It throws on the current code and passes with the fix. The full suite stays green (29 tests).