Fix bug that vectors with index 0 can't be drawn.

(Taken from https://github.com/open-craft/jsinput-vectordraw/pull/24/commits/33c1d0cf53806b5fa67b290293af2f8e5b53a1e9)

vectordraw/public/js/vectordraw.js

@@ -422,7 +422,7 @@ function VectorDrawXBlock(runtime, element, init_args) {
         var selected = this.getSelectedElement();
         var coords = this.getMouseCoords(evt);
         var targetObjects = this.objectsUnderMouse(coords);
-        if (selected.idx && (!targetObjects || _.all(targetObjects, this.canCreateVectorOnTopOf.bind(this)))) {
+        if (!_.isEmpty(selected) && (!targetObjects || _.all(targetObjects, this.canCreateVectorOnTopOf.bind(this)))) {
             var point_coords = [coords.usrCoords[1], coords.usrCoords[2]];
             if (selected.type === 'vector') {
                 this.drawMode = true;