fix(Tinymce): When content is null,set content to '' to avoid error (PanJiaChen#732)

Insua · PanJiaChen · commit a7942636c608 · 2018-06-01T15:38:08.000+08:00
When get rich editor value(null) from backend,it will occur error
```
Error in nextTick: "TypeError: Cannot read property 'replace' of null"
```
So I set val to '' to avoid it
diff --git a/src/components/Tinymce/index.vue b/src/components/Tinymce/index.vue
@@ -50,7 +50,8 @@ export default {
   watch: {
     value(val) {
       if (!this.hasChange && this.hasInit) {
-        this.$nextTick(() => window.tinymce.get(this.tinymceId).setContent(val))
+        this.$nextTick(() =>
+          window.tinymce.get(this.tinymceId).setContent(val || ''))
       }
     }
   },

Original file line number	Diff line number	Diff line change
`@@ -50,7 +50,8 @@ export default {`
`50`	`50`	`watch: {`
`51`	`51`	`value(val) {`
`52`	`52`	`if (!this.hasChange && this.hasInit) {`
`53`		`- this.$nextTick(() => window.tinymce.get(this.tinymceId).setContent(val))`
	`53`	`+ this.$nextTick(() =>`
	`54`	`+ window.tinymce.get(this.tinymceId).setContent(val \|\| ''))`
`54`	`55`	`}`
`55`	`56`	`}`
`56`	`57`	`},`