Null check on Picker Items (#24057)

Summary:
when conditional rendering is used then picker breaks when the child is null in iOS

This conditional rendering inside Picker fails when "someBooleanValue" variable is false

```
{
   this.state.someBooleanValue && <Picker.Item label="value" value="value" />
}
```

[iOS] [Fixed] - null check on children by using filter
Pull Request resolved: #24057

Differential Revision: D14538337

Pulled By: cpojer

fbshipit-source-id: d9324671931b5f1dac65d8058d9aa957b650af25

Libraries/Components/Picker/PickerIOS.ios.js

@@ -91,16 +91,18 @@ class PickerIOS extends React.Component<Props, State> {
   static getDerivedStateFromProps(props: Props): State {
     let selectedIndex = 0;
     const items = [];
-    React.Children.toArray(props.children).forEach(function(child, index) {
-      if (child.props.value === props.selectedValue) {
-        selectedIndex = index;
-      }
-      items.push({
-        value: child.props.value,
-        label: child.props.label,
-        textColor: processColor(child.props.color),
+    React.Children.toArray(props.children)
+      .filter(child => child !== null)
+      .forEach(function(child, index) {
+        if (child.props.value === props.selectedValue) {
+          selectedIndex = index;
+        }
+        items.push({
+          value: child.props.value,
+          label: child.props.label,
+          textColor: processColor(child.props.color),
+        });
       });
-    });
     return {selectedIndex, items};
   }