do not push on undefined no not close the stream

Currently if the mapper returns `null` or `undefined` the stream will end since `push(null)` and `push(undefined)` closes streams in streams2.

This breaks API compatibility with the split module where you would often use this pattern

``` js
split(function(line) {
  if (line) return map(line)
})
``` 

This PR fixes this by not forwarding undefined to push. It's up to you if you want to not forward null as well.