fixed issue with missing rewards crashing the app

This is more of a bandaide then a fix, https://github.com/Applifier/unity-ads/issues/4.  I say that because if the UnityAdsRewardItem really is missing, like has nil keys instead of just empty, the app still crashes just further down stream (in UnityAdsViewStateVideoPlayer.m, videoPlayerStartedPlaying).  If you are going to return nil from methods the entire calling chain needs to handle that and fail gracefully.  That is more work then im ready to tackle tonight.  Happy to jump on a skype call and talk through this with anyone.

Here are the changes I made:

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 checking for length 0, because the app will crash downstream from the defaultItem in UnityAdsIncentivizedZone being nil, since defaultItem is a UnityAdsRewardItem.  However works (plays video and doesnt crash) if the UnityAdsRewardItem.key is empty just empty.
- 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.