Spell vamp now only affects champs with health costs

I feel like this would be a fair change to spell vamp. Spell vamp is meant for champs who actually need to spend health to use their abilities. I feel like this stat is toxic on manaless champs (like Kennen, Katarina, Akali, etc) as they don't need this stat and it just makes them even more of a pain to kill. They can already spam their abilities all day, they don't need more sustain. But someone like Vladimir or Mordekaiser can't just sit in land spamming their abilities all day. Anyways, let the champs who actually need this stat benefit from it, and stop letting energy based champions abuse it.