fix(event-flow-viz): handle null metadata selection (#11885)

* Initialize object with empty array if is multiselect, fix 500 BE error

* modify data coming

superset-frontend/src/explore/components/controls/SelectControl.jsx

@@ -102,10 +102,9 @@ export default class SelectControl extends React.PureComponent {
   // Beware: This is acting like an on-click instead of an on-change
   // (firing every time user chooses vs firing only if a new option is chosen).
   onChange(opt) {
-    let optionValue = null;
+    let optionValue = this.props.multi ? [] : null;
     if (opt) {
       if (this.props.multi) {
-        optionValue = [];
         opt.forEach(o => {
           // select all options
           if (o.meta === true) {