do not null slice pointers by default

[davidnewhall]

(Struct | null)[] sucks. This fixes it. If you like it the other way, pass true to the new override.

Example:

	goat := goty.NewGoty(&goty.Config{
		Docs: docs,
		GlobalOverrides: goty.Override{
			NullSlicePointers: true,
		},
		Overrides: goty.Overrides{
			cnfg.Duration{}:                 {Type: "string"},
			reflect.TypeOf(time.Weekday(0)): {Comment: "The day of the week."},
		},
	})