st/icon: Always show empty texture if both gicons are NULL

Commit 7ff7fb5d3b forgot to clear the
`priv->icon_texture` actor when returning from st_icon_update(), which
means we don't always switch to an empty icon if both gicon properties
are set to NULL.

Fix this and destroy the actor before returning early from
st_icon_update().

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1178


(cherry picked from commit 07deda593a)

src/st/st-icon.c

@@ -419,7 +419,10 @@ st_icon_update (StIcon *icon)
     }
 
   if (priv->gicon == NULL && priv->fallback_gicon == NULL)
+    {
+      g_clear_pointer (&priv->icon_texture, clutter_actor_destroy);
       return;
+    }
 
   if (!st_widget_get_resource_scale (ST_WIDGET (icon), &resource_scale))
     return;