fix maintain attrs hook

assets/js/maintain_attrs.js

@@ -2,7 +2,10 @@
 // update. https://github.com/phoenixframework/phoenix_live_view/issues/1011
 
 export default {
-  attrs () { return this.el.getAttribute('data-attrs').split(', ') },
+  attrs() {
+    const attrs = this.el.getAttribute('data-attrs');
+    if (attrs) { return attrs.split(', '); } else { return []; }
+  },
   beforeUpdate () { this.prevAttrs = this.attrs().map(name => [name, this.el.getAttribute(name)]) },
   updated () { this.prevAttrs.forEach(([name, val]) => this.el.setAttribute(name, val)) }
 }