Fix breaking of PickerAndroid when child is null. (#23748)

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

This conditional rendering inside Picker fails when a is 1, because child will be null in PickerAndroid.android.js.
```
{
   this.state.a === 2 && <Picker.Item label="value" value="value" />
}
```

[ANDROID] [FIXED] - When child is null it returns.
Pull Request resolved: https://github.com/facebook/react-native/pull/23748

Differential Revision: D14319140

Pulled By: cpojer

fbshipit-source-id: 07d48cd054a131e6984cfe3bdce7538ffbc50622

Libraries/Components/Picker/PickerAndroid.android.js

@@ -64,6 +64,9 @@ class PickerAndroid extends React.Component<
   ): PickerAndroidState {
     let selectedIndex = 0;
     const items = React.Children.map(props.children, (child, index) => {
+      if (child === null) {
+        return;
+      }
       if (child.props.value === props.selectedValue) {
         selectedIndex = index;
       }