Fix iPad crash issue of the actionsheet

[sungjin-woo]

This PR is related to #20 issue.

When I show actionsheet on iPad using the following method, my app was always crashed without passing the popoverPresentationControllerBlock.

+ (instancetype)showActionSheetInViewController:(UIViewController *)viewController
                                      withTitle:(NSString *)title
                                        message:(NSString *)message
                              cancelButtonTitle:(NSString *)cancelButtonTitle
                         destructiveButtonTitle:(NSString *)destructiveButtonTitle
                              otherButtonTitles:(NSArray *)otherButtonTitles
             popoverPresentationControllerBlock:(void(^)(RMPopoverPresentationController *popover))popoverPresentationControllerBlock
                                       tapBlock:(RMUniversalAlertCompletionBlock)tapBlock;

So if the popoverPresentationControllerBlock is passed by NULL ,
I want to show the actionsheet on the center of parent view controller's view like UIActionSheet.

Please consider this PR.
Thank you.