fixed issue with missing rewards crashing the app

This fixes this issue https://github.com/Applifier/unity-ads/issues/4

in UnityAdsRewardItem.m
-Removed the expsensive exception from UnityAdsRewardItem and instead just return nil.  Exceptions should only be thrown from very serious errors, a missing key isn't serious. 
-Removed redundant nil checks, calling methods on nil objects does nothing so [nil length] is 0.
- switched methods to modern ObjC syntax

UnityAdsIncentivizedZone.m
- if one of the UnityAdsRewardItem is null, just continue, this fixes the crash.  Ads still show just fine in my testing.
- did some syntax cleanup, made property readonly instead of having the method.