fix(ssr): reference error when create $ssrContext for root component (#5981)

fix #5941

Author: jkzing

src/core/index.js

@@ -11,7 +11,7 @@ Object.defineProperty(Vue.prototype, '$isServer', {
 Object.defineProperty(Vue.prototype, '$ssrContext', {
   get () {
     /* istanbul ignore next */
-    return this.$vnode.ssrContext
+    return this.$vnode && this.$vnode.ssrContext
   }
 })
 

test/ssr/ssr-basic-renderer.spec.js

@@ -50,4 +50,21 @@ describe('SSR: basicRenderer', () => {
       done()
     })
   })
+
+  // #5941
+  it('should work peoperly when accessing $ssrContext in root component', done => {
+    let ssrContext
+    renderToString(new Vue({
+      template: `
+        <div></div>
+      `,
+      created () {
+        ssrContext = this.$ssrContext
+      }
+    }), (err, result) => {
+      expect(err).toBeNull()
+      expect(ssrContext).toBeUndefined()
+      done()
+    })
+  })
 })