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

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

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 || ''))
       }
     }
   },