incusd/storage/zfs: Fix version handling

Signed-off-by: Stéphane Graber <stgraber@stgraber.org>

incusd/storage/drivers/driver_zfs_utils.go

@@ -285,7 +285,7 @@ func (d *zfs) version() (string, error) {
 	// This function is only really ever relevant on Ubuntu as the only
 	// distro that ships out of sync tools and kernel modules
 	out, err := shared.RunCommand("dpkg-query", "--showformat=${Version}", "--show", "zfsutils-linux")
-	if err == nil {
+	if out != "" && err == nil {
 		return strings.TrimSpace(string(out)), nil
 	}