properties: Fix opening gnome-disks for root filesystem

With the introduction of nautilus-dbus-launcher, we made it so that
the current disk is selected in gnome-disks (when opening from the
properties window) by passing the disk identifier to --block-device.
This works fine for most mounted volumes, but breaks the ability to
open gnome-disks for the root filesystem because
g_file_find_enclosing_mount() doesn't return a mount for the root file
system (it only returns "user interesting locations").  Rather than
creating a more complicated setup, use the old behavior for the root
filesystem where no volume is selected by default.

Fixes: #2447

src/nautilus-properties-window.c

@@ -2613,12 +2613,20 @@ open_in_disks (NautilusPropertiesWindow *self)
 
     mount = nautilus_file_get_mount (get_original_file (self));
     volume = (mount != NULL) ? g_mount_get_volume (mount) : NULL;
-    device_identifier = g_volume_get_identifier (volume,
-                                                 G_VOLUME_IDENTIFIER_KIND_UNIX_DEVICE);
 
-    parameters = g_variant_new_parsed ("(objectpath '/org/gnome/DiskUtility', "
-                                       "@aay [], {'options': <{'block-device': <%s>}> })",
-                                       device_identifier);
+    if (volume != NULL)
+    {
+        device_identifier = g_volume_get_identifier (volume,
+                                                     G_VOLUME_IDENTIFIER_KIND_UNIX_DEVICE);
+
+        parameters = g_variant_new_parsed ("(objectpath '/org/gnome/DiskUtility', "
+                                           "@aay [], {'options': <{'block-device': <%s>}> })",
+                                           device_identifier);
+    }
+    else
+    {
+        parameters = g_variant_new_parsed ("(objectpath '/org/gnome/DiskUtility', @aay [], @a{sv} {})");
+    }
 
     nautilus_dbus_launcher_call (launcher,
                                  NAUTILUS_DBUS_LAUNCHER_DISKS,