Fix: prefer-output-null crashes, when a test case is not object.

Author: aladdin-add

lib/rules/prefer-output-null.js

@@ -40,8 +40,11 @@ module.exports = {
             * @returns {Node} found node; if not found, return null;
             */
             function getTestInfo (key) {
-              const res = test.properties.filter(item => item.key.name === key);
-              return res.length ? res[res.length - 1] : null;
+              if (test.type === 'ObjectExpression') {
+                const res = test.properties.filter(item => item.key.name === key);
+                return res.length ? res[res.length - 1] : null;
+              }
+              return key === 'code' ? test : null;
             }
 
             const code = getTestInfo('code');