Support updating values in empty documents

pkg/utils/yaml_utils/yaml_utils.go

@@ -15,6 +15,13 @@ func UpdateYamlValue(yamlBytes []byte, path []string, value string) ([]byte, err
 		return nil, fmt.Errorf("failed to parse YAML: %w", err)
 	}
 
+	// Empty document: need to create the top-level map ourselves
+	if len(node.Content) == 0 {
+		node.Content = append(node.Content, &yaml.Node{
+			Kind: yaml.MappingNode,
+		})
+	}
+
 	body := node.Content[0]
 
 	updateYamlNode(body, path, value)

pkg/utils/yaml_utils/yaml_utils_test.go

@@ -31,6 +31,14 @@ func TestUpdateYamlValue(t *testing.T) {
 			expectedOut: "foo: bar\nfoo2: baz\n",
 			expectedErr: "",
 		},
+		{
+			name:        "add new key and value when document was empty",
+			in:          "",
+			path:        []string{"foo"},
+			value:       "bar",
+			expectedOut: "foo: bar\n",
+			expectedErr: "",
+		},
 		{
 			name:        "preserve inline comment",
 			in:          "foo: bar # my comment\n",