Shutup LinkPreviewDelegate

Make sure that the console isn't spammed having `linkpreviewer` undefined in `LinkPreviewDelegate`, even though it shouldn't ever be and I couldn't find any case where it was but my console was still intermittently getting spammed.

src/linkpreviewer.cpp

@@ -93,4 +93,9 @@ void LinkPreviewer::loadUrlPreview()
     }
 }
 
+bool LinkPreviewer::empty() const
+{
+    return m_url.isEmpty();
+}
+
 #include "moc_linkpreviewer.cpp"