i3-sway: Empty set argument was passed to wrong function (#2819)

This would give the error "attempt to call something which is not a
function but a list" given that `optionals a b` returns a list. `indent`
is the one taking this empty set as second argument.

modules/services/window-managers/i3-sway/lib/functions.nix

@@ -123,7 +123,7 @@ rec {
                   (optionalString (colors.bindingMode != null)
                     "binding_mode ${barColorSetStr colors.bindingMode}")
                   "}"
-                ])) { })
+                ]) { }))
               extraConfig
             ])) { })
         }