add nullability type annotations to callback blocks

Author: ahmednawar

Branch-SDK/Branch-SDK/BranchUniversalObject.h

@@ -11,8 +11,8 @@
 
 @class BranchLinkProperties;
 
-typedef void (^callback) ();
-typedef void (^shareCompletion) (NSString *activityType, BOOL completed);
+typedef void (^_Nullable callback) (); // TO BE REMOVED: I don't this called anywhere, shall I remove it?
+typedef void (^shareCompletion) (NSString *_Nonnull activityType, BOOL completed);
 
 typedef NS_ENUM(NSInteger, ContentIndexMode) {
     ContentIndexModePublic,