Fixed crash caused when parsing an url with a target_url property set…

Created by: yuun

… to null in applinks resulting in a NSNull object improperly considered as valid NSString

I had this issue with Facebook hosted applinks returning a null target_url like described below bolts://?al_applink_data={"user_agent":"Bolts iOS 1.0.0","target_url":null}

Note that in this case, null value is resulting in a NSNull object by NSJSONSerialization parser.