IDL compiler: move EventHandler null check out of generated bindings

Per request at:
https://codereview.chromium.org/30493002/#msg4
...this moves some code out of generated bindings and into bindings/v8.

BUG=239771

Review URL: https://codereview.chromium.org/32323012

git-svn-id: svn://svn.chromium.org/blink/trunk@160183 bbb929c8-8fbe-4397-9dbb-9b2b20218538