Fixed a release notes icon bug when installing/cancelling and update

The release notes icon would appear when canelling an update of an application that doew nost have a description (and thus the icon should not appear)

src/Widgets/AppContainers/InstalledPackageRowGrid.vala

@@ -122,7 +122,9 @@ public class AppCenter.Widgets.InstalledPackageRowGrid : AbstractPackageRowGrid
                     release_button_revealer.reveal_child = true;
                 }
             } else {
-                release_button_revealer.reveal_child = true;
+                if (newest.get_description () != null) {
+                    release_button_revealer.reveal_child = true;
+                }
             }
         }