Setup Matomo only with endpoint url and site id

See issue #29

packages/matomo/src/matomo.js

@@ -41,6 +41,14 @@ export class MatomoElement extends DBPLitElement {
 
     setupMatomo(loggedIn) {
         if (loggedIn && ! this.isRunning) {
+            if (this.siteId === -1) {
+                console.log('site id missing, skipping matomo...');
+                return;
+            }
+            if (this.endpoint === '') {
+                console.log('endpoint missing, skipping matomo...');
+                return;
+            }
             console.log('add matomo...');
 
             window._paq = window._paq || [];