Make linkEvent return undefined instead of null

[as-com]

Fixes #1310

This time the branch is correct. 😄

[coveralls]

Coverage remained the same at 95.807% when pulling 0d2f9d8 on as-com:fix-1310 into 24f97a3 on infernojs:dev.

[Havunen]

I dont know if returning undefined is good thing, generally we want to avoid those because v8 dont like them. Also adding null value to event listener should be allowed that way you can clear the listener fe: having boolean condition and then return null if you want to remove it.

[as-com]

Closing in favor of #1313