要检查myHero是否为空

[Eroica-cpp]

https://github.com/k5dash/LuaDota2/blob/master/AbilityAlertAdvanced.lua#L430
https://github.com/k5dash/LuaDota2/blob/master/AbilityAlertAdvanced.lua#L472

要检查myHero是否为空，不然观战或者看replay的时候会出错。