Add unit test

test/at-rule.test.ts

@@ -48,4 +48,10 @@ test('clone spaces from another at-rule', () => {
   is(rule.toString(), '@page 1{}')
 })
 
+test('at-rule without body has no nodes property', () => {
+  let root = parse('@layer a, b, c;');
+  let layer = root.first as AtRule
+  type(layer.nodes, 'undefined')
+});
+
 test.run()