Go: Fix parsing of nullable colors

Fixes #6629

kittens/diff/render.go

@@ -169,6 +169,7 @@ func create_formatters() {
 		return ans
 	}
 	format_as_sgr.filler = only_open("bg=" + conf.Filler_bg.AsRGBSharp())
+	debugprintln(11111, conf.Margin_filler_bg.IsSet)
 	if conf.Margin_filler_bg.IsSet {
 		format_as_sgr.margin_filler = only_open("bg=" + conf.Margin_filler_bg.Color.AsRGBSharp())
 	} else {

tools/utils/style/wrapper.go

@@ -172,7 +172,7 @@ func ParseColorOrNone(color string) (NullableColor, error) {
 		return NullableColor{}, nil
 	}
 	c, err := ParseColor(raw)
-	return NullableColor{Color: c}, err
+	return NullableColor{Color: c, IsSet: err == nil}, err
 }
 
 var named_colors = map[string]uint8{