More robust GPX parsing

src/lib/gpx-parser-builder/src/link.js

@@ -1,8 +1,8 @@
 export default class Link {
   constructor(object) {
     this.$ = {};
-    this.$.href = object.$.href || object.href;
+    this.$.href = object.$?.href || object.href;
     this.text = object.text;
     this.type = object.type;
   }
-}
+}