#642 Use == null instead == undefined

Originally at 2013-08-29T12:49:46Z by Herbert Vojčík closed at 2013-09-09T12:43:09Z `== null` is used idiom to compare to `undefined` and `null` at once and is shorter (and marginally safer, since `null` is keyword and `undefined` is potentially redefinable global variable). At least in `ifNil` family `== undefined` is used.