test: fix tests for preserving ruleNames

packages/@vue/cli-service/__tests__/Service.spec.js

@@ -233,6 +233,28 @@ test('api: configureWebpack returning object', () => {
   expect(config.output.path).toBe('test-dist-3')
 })
 
+test('api: configureWebpack preserve ruleNames', () => {
+  const service = createMockService([
+    {
+      id: 'babel',
+      apply: require('@vue/cli-plugin-babel')
+    },
+    {
+      id: 'test',
+      apply: api => {
+        api.configureWebpack({
+          module: {
+            rules: []
+          }
+        })
+      }
+    }
+  ])
+
+  const config = service.resolveWebpackConfig()
+  expect(config.module.rules[0].__ruleNames).toEqual(['js'])
+})
+
 test('api: configureDevServer', () => {
   const cb = () => {}
   const service = createMockService([{

packages/@vue/cli-service/lib/Service.js

@@ -234,8 +234,8 @@ module.exports = class Service {
     // vue inspect works properly.
     if (config !== original) {
       cloneRuleNames(
-        config.module.rules,
-        original.module.rules
+        config.module && config.module.rules,
+        original.module && original.module.rules
       )
     }
 
@@ -352,14 +352,15 @@ function removeSlash (config, key) {
 }
 
 function cloneRuleNames (to, from) {
+  if (!to || !from) {
+    return
+  }
   from.forEach((r, i) => {
     if (to[i]) {
       Object.defineProperty(to[i], '__ruleNames', {
         value: r.__ruleNames
       })
-      if (to[i].oneOf && r.oneOf) {
-        cloneRuleNames(to[i].oneOf, r.oneOf)
-      }
+      cloneRuleNames(to[i].oneOf, r.oneOf)
     }
   })
 }