Fix image cannot show in iOS 14 (facebook#29420)

Summary:
This PR is to fix facebook#29279, which image cannot show in iOS 14
As facebook#29279 (comment) mention, this issue can be fixed by calling ` [super displayLayer:layer];` it it is still image, to let `UIImageView` handle still image rendering

## Changelog

[iOS] [Fixed] - Fix image cannot show in iOS 14

Pull Request resolved: facebook#29420

Test Plan:
Image can be shown in iOS 14 build with Xcode 12 beta, using
```js
<Image source={require('./images/some_local_image.jpg')}/>
```

It may also need to test gif image is render correctly
```js
<Image source={{uri: 'https://some_remote_gif_image.gif'}}/>
```

Reviewed By: p-sun

Differential Revision: D22619448

Pulled By: shergin

fbshipit-source-id: f4d0ad83af945a6b8099d4eaea5a5f1933c7bfd2

Libraries/Image/RCTUIImageViewAnimated.m

@@ -278,6 +278,8 @@ - (void)displayLayer:(CALayer *)layer
   if (_currentFrame) {
     layer.contentsScale = self.animatedImageScale;
     layer.contents = (__bridge id)_currentFrame.CGImage;
+  } else {
+    [super displayLayer:layer];
   }
 }