fix(ssr): textarea keeps undefined/null values (#11121)

* fix(ssr): textarea keeps undefined/null values

* refactor(ssr): Code review changes for null values in textareas

src/platforms/web/server/modules/dom-props.js

@@ -2,7 +2,7 @@
 
 import VNode from 'core/vdom/vnode'
 import { renderAttr } from './attrs'
-import { isDef, isUndef, extend } from 'shared/util'
+import { isDef, isUndef, extend, toString } from 'shared/util'
 import { propsToAttrMap, isRenderableAttr } from '../util'
 
 export default function renderDOMProps (node: VNodeWithData): string {
@@ -28,7 +28,7 @@ export default function renderDOMProps (node: VNodeWithData): string {
     } else if (key === 'textContent') {
       setText(node, props[key], false)
     } else if (key === 'value' && node.tag === 'textarea') {
-      setText(node, props[key], false)
+      setText(node, toString(props[key]), false)
     } else {
       // $flow-disable-line (WTF?)
       const attr = propsToAttrMap[key] || key.toLowerCase()

test/ssr/ssr-string.spec.js

@@ -1594,6 +1594,25 @@ describe('SSR: renderToString', () => {
 
     renderToString(vueInstance, err => done(err))
   })
+
+  it('undefined v-model with textarea', done => {
+    renderVmWithOptions({
+      render (h) {
+        return h('div', [
+          h('textarea', {
+            domProps: {
+              value: null
+            }
+          })
+        ])
+      }
+    }, result => {
+      expect(result).toContain(
+        '<div data-server-rendered="true"><textarea></textarea></div>'
+      )
+      done()
+    })
+  })
 })
 
 function renderVmWithOptions (options, cb) {