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: #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;
  }