fix(index): check node.selector

Author: akameco

index.js

@@ -18,7 +18,9 @@ module.exports = (input /*: string */) /*: Set<string> */ => {
 
   const result = parser.parse(input)
   for (const node of result.nodes) {
-    selectorParser(transform).processSync(node.selector)
+    if (node.selector) {
+      selectorParser(transform).processSync(node.selector)
+    }
   }
 
   return set

test.js

@@ -10,6 +10,10 @@ test('return class names', () => {
     .testCase {
       color: white;
     }
+    @keyframes App-logo-spin {
+      from { transform: rotate(0deg); }
+      to { transform: rotate(360deg); }
+    }
   `
   expect(m(css)).toEqual(new Set(['a', 'b', 'c', 'testCase']))
 })