checker: clarify error message about using empty map literal to initialize structs

[StunxFS]

This PR clarifies the error message about using empty map literal to initialize structs, in addition to that it prevents another error message from appearing when using empty map literal with anonymous structs.

Fix #22498.

$ cat main.v
struct AStructName {
        foo  int
        anon struct {
                bar int
        }
}

fn main() {
        s := AStructName{5, {}}
        println(s)
}

$ v main.v
main.v:9:22: error: `{}` cannot be used to initialize anonymous structs. Use `struct{}` instead.
    7 | 
    8 | fn main() {
    9 |     s := AStructName{5, {}}
      |                         ~~
   10 |     println(s)
   11 | }

[spytheman]

Excellent work.