fixed #2396: don't always add p-splitter-panel-nested class (#2394)

p-splitter-panel-nested class is always added no matter whether the panel element contains children with 'p-splitter' or not.

src/components/splitter/Splitter.js

@@ -215,7 +215,7 @@ export class Splitter extends Component {
 
     componentDidMount() {
         if (this.panelElement) {
-            if (this.panelElement.childNodes && DomHandler.find(this.panelElement, '.p-splitter')) {
+            if (this.panelElement.childNodes && DomHandler.find(this.panelElement, '.p-splitter').length) {
                 DomHandler.addClass(this.panelElement, 'p-splitter-panel-nested');
             }
         }