fix type_default for empty structs

vlib/v/gen/c/cgen.v

@@ -7574,8 +7574,9 @@ fn (mut g Gen) type_default_impl(typ_ ast.Type, decode_sumtype bool) string {
 			}
 		}
 		.struct {
-			mut has_none_zero := false
 			info := sym.info as ast.Struct
+			mut has_none_zero := info.fields.len == 0
+
 			mut init_str := if info.is_anon && !g.inside_global_decl {
 				'(${g.styp(typ)}){'
 			} else {

vlib/x/json2/types.v

@@ -24,9 +24,7 @@ pub type Any = []Any
 	| Null
 
 // Null is a simple representation of the `null` value in JSON.
-pub struct Null {
-	is_null bool = true
-}
+pub struct Null {}
 
 // null is an instance of the Null type, to ease comparisons with it.
 pub const null = Null{}