Fix opening empty browser window when clicking osm marker

public/typo3conf/ext/ep_theme/Resources/Private/Assets/js/controllers/osm_controller.js

@@ -20,7 +20,7 @@ export default class extends Controller {
         if (this.hasLatValue && this.hasLngValue) {
             const position = L.latLng(this.latValue, this.lngValue)
             const marker = L.marker(position)
-            if (this.hasExternalLinkValue) {
+            if (this.hasExternalLinkValue && '' !== this.externalLinkValue) {
                 marker.on('click', () => {
                     window.open(this.externalLinkValue, '_blank')
                 })