x/exp/schema: make zero value of Schema work as expected

Signed-off-by: Phil Hassey <phil@strongdm.com>

Author: philhassey

x/exp/schema/schema.go

@@ -26,7 +26,7 @@ func (s *Schema) SetFilename(filename string) {
 
 // MarshalJSON encodes the Schema in the JSON format.
 func (s *Schema) MarshalJSON() ([]byte, error) {
-	jsonSchema := (*json.Schema)(s.schema)
+	jsonSchema := (*json.Schema)(s.astOrEmpty())
 	return jsonSchema.MarshalJSON()
 }
 
@@ -42,7 +42,7 @@ func (s *Schema) UnmarshalJSON(b []byte) error {
 
 // MarshalCedar encodes the Schema in the human-readable format.
 func (s *Schema) MarshalCedar() ([]byte, error) {
-	return parser.MarshalSchema(s.schema), nil
+	return parser.MarshalSchema(s.astOrEmpty()), nil
 }
 
 // UnmarshalCedar parses a Schema in the human-readable format.
@@ -57,10 +57,17 @@ func (s *Schema) UnmarshalCedar(b []byte) error {
 
 // AST returns the underlying AST.
 func (s *Schema) AST() *ast.Schema {
-	return s.schema
+	return s.astOrEmpty()
 }
 
 // Resolve returns a resolved.Schema with type references resolved and declarations indexed.
 func (s *Schema) Resolve() (*resolved.Schema, error) {
-	return resolved.Resolve(s.schema)
+	return resolved.Resolve(s.astOrEmpty())
+}
+
+func (s *Schema) astOrEmpty() *ast.Schema {
+	if s.schema == nil {
+		return &ast.Schema{}
+	}
+	return s.schema
 }

x/exp/schema/schema_test.go

@@ -720,6 +720,25 @@ func TestSchema(t *testing.T) {
 		testutil.Error(t, err)
 	})
 
+	t.Run("ZeroValueSchema", func(t *testing.T) {
+		t.Parallel()
+		var s schema.Schema
+
+		b, err := s.MarshalCedar()
+		testutil.OK(t, err)
+		testutil.Equals(t, string(b), "")
+
+		jb, err := s.MarshalJSON()
+		testutil.OK(t, err)
+		testutil.Equals(t, string(jb), "{}")
+
+		r, err := s.Resolve()
+		testutil.OK(t, err)
+		testutil.Equals(t, r != nil, true)
+
+		testutil.Equals(t, s.AST() != nil, true)
+	})
+
 	t.Run("EmptySchema", func(t *testing.T) {
 		t.Parallel()
 		s := schema.NewSchemaFromAST(&ast.Schema{})