internal/encoding/yaml: attach a filename position to an empty file "null"

As otherwise any reported errors are missing the filename.
There is no line or column information, which is fine, as this is an
empty YAML file where the null was not actually spelled out.

Seems to slightly improve a JSON Schema decoding error as well.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Iadee9aea7431786fbdfeddb25d2b6a31a77a6f4f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1211414
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

cmd/cue/cmd/testdata/script/empty_yaml.txtar

@@ -13,7 +13,7 @@ cmp stderr export-all.stderr
 null
 -- export-all.stderr --
 conflicting values null and {two:"two text"} (mismatched types null and struct):
-    
+    ./empty.yaml
     ./two.yaml:1:1
 -- one.yaml --
 one: "one text"

encoding/jsonschema/testdata/txtar/openapi_nonexistent_error.txtar

@@ -6,4 +6,5 @@ Test what happens when there's an OpenAPI schema that has no
 -- schema.yaml --
 -- out/decode/extract --
 ERROR:
-root value at path #/components/schemas/ does not exist
+root value at path #/components/schemas/ does not exist:
+    schema.yaml

internal/encoding/yaml/decode.go

@@ -108,8 +108,9 @@ func (d *decoder) Decode() (ast.Expr, error) {
 			// with a null scalar value inside instead.
 			if !d.yamlNonEmpty {
 				return &ast.BasicLit{
-					Kind:  token.NULL,
-					Value: "null",
+					Kind:     token.NULL,
+					ValuePos: d.tokFile.Pos(0, token.NoRelPos),
+					Value:    "null",
 				}, nil
 			}
 			// If the input wasn't empty, we already decoded some CUE syntax nodes,