updated animatedImage setter for fixing image disappear issue

If we set still UIImage more than once the image will disappear.
The `setImage` method will call setAnimatedImage which casing this problem.
The reason is that calling `setNeedDisplay` the existing content in the layer will be removed it and original code doesn't handle nil case so it pass checking.
```
- (void)setNeedsDisplay
Description	
Marks the layer’s contents as needing to be updated.
Calling this method causes the layer to recache its content. This results in the layer potentially calling either the displayLayer: or drawLayer:inContext: method of its delegate. The existing content in the layer’s contents property is removed to make way for the new content.
```