plug-ins: Allow Python GUI resources to be empty

Resolves #11772
While ba79fd00 fixed the number of
arguments in the resource arguments,
setting them to False prevented the GUI
from opening if the gradient wasn't set.
This patch switches the value to True to
fix the problem.

plug-ins/python/palette-offset.py

@@ -57,7 +57,7 @@ class PaletteOffset (Gimp.PlugIn):
                                          Gimp.RunMode.NONINTERACTIVE,
                                          GObject.ParamFlags.READWRITE)
             procedure.add_palette_argument ("palette", _("_Palette"),
-                                            _("Palette"), False,
+                                            _("Palette"), True,
                                             GObject.ParamFlags.READWRITE)
             procedure.add_int_argument ("amount", _("O_ffset"), _("Offset"),
                                         1, GLib.MAXINT, 1, GObject.ParamFlags.READWRITE)