Do not append subtitle to document title when empty

src/App.vue

@@ -186,7 +186,7 @@ export default {
 
       document.title =
         this.config.documentTitle ||
-        `${this.config.title} | ${this.config.subtitle}`;
+        [this.config.title, this.config.subtitle].filter(Boolean).join(" | ");
       if (this.config.stylesheet) {
         let stylesheet = "";
         let addtionnal_styles = this.config.stylesheet;