Fix missing nullability in PickerManager.setNativeSelectedIndex

Summary:
Changelog: [Internal]

To ensure compatibility with Android, nullability needs to be specified.

Reviewed By: shergin

Differential Revision: D18590634

fbshipit-source-id: 9cd969f4f187e8f1d004f03692dd9b29c4f37f95

React/Views/RCTPickerManager.m

@@ -43,7 +43,7 @@ RCT_CUSTOM_VIEW_PROPERTY(fontFamily, NSString, RCTPicker)
   view.font = [RCTFont updateFont:view.font withFamily:json ?: defaultView.font.familyName];
 }
 
-RCT_EXPORT_METHOD(setNativeSelectedIndex : (nonnull NSNumber *)viewTag toIndex : (NSNumber *)index)
+RCT_EXPORT_METHOD(setNativeSelectedIndex : (nonnull NSNumber *)viewTag toIndex : (nonnull NSNumber *)index)
 {
   [self.bridge.uiManager addUIBlock:^(RCTUIManager *uiManager, NSDictionary<NSNumber *, UIView *> *viewRegistry) {
     UIView *view = viewRegistry[viewTag];