Decode null as NULL token in yaml decoder

This PR will fix #185 as `BasicLit` will be returned for null value.

When we walk through the ast to create the instance, null value will be handled as `BasicLit` instead of `Ident`.

Closes #186
#186

GitOrigin-RevId: 9dadc29
Change-Id: I8bfd84c5c1c4ab60bbc1fbfcd76bb5a4561a8237
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4100
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

internal/third_party/yaml/decode.go

@@ -443,7 +443,11 @@ func (d *decoder) scalar(n *node) ast.Expr {
 		}
 	}
 	if resolved == nil {
-		return d.ident(n, "null")
+		return &ast.BasicLit{
+			ValuePos: d.start(n),
+			Kind:     token.NULL,
+			Value:    "null",
+		}
 	}
 	switch tag {
 	// TODO: use parse literal or parse expression instead.