fix(geojsonSource): fill data with empty FeatureCollection

It seems that source.setData don't do anything if the source if empty. This fix handle cases when you create a source without immediate data.

src/app/lib/source/geojson/geojson-source.component.ts

@@ -35,6 +35,12 @@ export class GeoJSONSourceComponent implements OnInit, OnDestroy, OnChanges, Geo
   ) { }
 
   ngOnInit() {
+    if (!this.data) {
+      this.data = {
+        type: 'FeatureCollection',
+        features: []
+      };
+    }
     this.MapService.mapLoaded$.subscribe(() => {
       this.MapService.addSource(this.id, {
         type: 'geojson',
@@ -81,12 +87,6 @@ export class GeoJSONSourceComponent implements OnInit, OnDestroy, OnChanges, Geo
   }
 
   addFeature(feature: GeoJSON.Feature<GeoJSON.GeometryObject>) {
-    if (!this.data) {
-      this.data = {
-        type: 'FeatureCollection',
-        features: []
-      };
-    }
     const collection = <GeoJSON.FeatureCollection<GeoJSON.GeometryObject>>this.data;
     collection.features.push(feature);
     this.updateFeatureData.next();