feat: If no vocabulary has been learned, this should be told

frontend/src/pages/vocab.tsx

@@ -131,6 +131,15 @@ export default class VocabPage extends React.Component<IProps> {
         const { vocab } = this.props;
         return <div className="content">
             {vocab.map(this.vocabToCard)}
+            {
+                vocab.length === 0 ? (
+                    <div className="flex-parent">
+                        <Typography variant="display1">
+                            Noch keine Vokabeln gelernt
+                        </Typography>
+                    </div>
+                ) : undefined
+            }
             <div className="vocab-bottom-spacer" />
 
             <Dialog