Bugfix - Only add valid IOSPicker items.

Summary:
Fixes https://github.com/facebook/react-native/issues/9216.

As nickzuber describes in #9216, conditional `Picker.Item` elements will lead to exceptions downstream when the `Picker` attempts to construct the collection of items.

[In the picker source](a2fb703bbb/Libraries/Components/Picker/PickerIOS.ios.js (L48-L53)) we can see that `child.props` is accessed when `child` has the potential to be an invalid `React` element.

```js
ReactChildren.forEach(props.children, function (child, index) {
  if (child.props.value === props.selectedValue) {
    selectedIndex = index;
  }
  items.push({value: child.props.value, label: child.props.label});
});
```

This change ensures the incoming element is valid

```diff
ReactChildren.forEach(props.children, function (child, index) {
+ if (!React.isValidElement(child)) {
+   return;
+ }
  if (child.props.value === props.selectedValue) {
    selectedIndex = index;
  }
  items.
Closes https://github.com/facebook/react-native/pull/9243

Differential Revision: D3847514

Pulled By: spicyj

fbshipit-source-id: f46fbd4b0f81de7a92e1ca3e60b5ed15a9cbbf78

Libraries/Components/Picker/PickerIOS.ios.js

@@ -45,7 +45,7 @@ var PickerIOS = React.createClass({
   _stateFromProps: function(props) {
     var selectedIndex = 0;
     var items = [];
-    ReactChildren.forEach(props.children, function (child, index) {
+    ReactChildren.toArray(props.children).forEach(function (child, index) {
       if (child.props.value === props.selectedValue) {
         selectedIndex = index;
       }