A Go type is generated for the ASN.1 NULL type which equates the ASN.1 type assignment to the built-in Go bool type. The purpose of having a type for it is so that it can act as a placeholder to denote the presence or absense of this type within a constructed type. For example, if it used in an optional element or within a choice construct.
If a NULL type is referenced directly in an element within a constructed type, the Go bool type is referenced directly.