Added check for start date, return if start is nil.
Added check for start date, return if start is nil.

appnexus/mobile-sdk-ios,appnexus/mobile-sdk-ios,appnexus/mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,appnexus/mobile-sdk-ios,appnexus/mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios,appnexus/mobile-sdk-ios,8tracks/appnexus-mobile-sdk-ios