make feature tap detection work for features with a null id on ios (s…

…ame as android) (maplibre#357)

Feature Id nullable on android but not on iOS.

On android the feature id can be null which works fine in our app but on
iOS clicking on points of interest the reaction is the same as clicking
elsewhere on the map.

In the android code 
![Screenshot 2023-12-12 at 13 42
00](https://github.com/maplibre/flutter-maplibre-gl/assets/26603883/a532aa76-9ec4-4bf4-8da3-b673989a85e6)

null is a valid value. I adapted the iOS code to allow for the same
behaviour.

Co-authored-by: m0nac0 <58807793+m0nac0@users.noreply.github.com>

ios/Classes/MapboxMapController.swift

@@ -989,9 +989,9 @@ class MapboxMapController: NSObject, FlutterPlatformView, MGLMapViewDelegate, Ma
         let point = sender.location(in: mapView)
         let coordinate = mapView.convert(point, toCoordinateFrom: mapView)
 
-        if let feature = firstFeatureOnLayers(at: point), let id = feature.identifier {
+        if let feature = firstFeatureOnLayers(at: point) {
             channel?.invokeMethod("feature#onTap", arguments: [
-                        "id": id,
+                        "id": feature.identifier,
                         "x": point.x,
                         "y": point.y,
                         "lng": coordinate.longitude,