ast, fmt: fix formatting type decl with anon struct 2 (related #19356)

[yuyi98]

This PR fix formatting type decl with anon struct 2 (related #19356).

• Fix formatting type decl with anon struct 2.
• Change test.

vlib\v\fmt\tests\type_decl_with_anon_struct_keep.vv

pub type Unit1 = struct {}

pub type Unit2 = struct {
	name string
	age  int
}

pub const unit1 = Unit1{}

pub const unit2 = Unit2{'bob', 22}

fn main() {
	println(unit1)
	println(unit2)
}

[Delta456]

Should this be allowed?

pub type Unit1 = struct {}

[yuyi98]

I think it should be allowed, It's aliases of empty struct.