FilterEventsToRawDocumentStep#handleStartDocument assumes outputStream isn't null

Hi all, I noticed handleStartDocument assumes the outputStream isn't null when it tries to set it. Depending on the writer, this can lead to a null pointer.

net/sf/okapi/steps/common/FilterEventsToRawDocumentStep.java:153

I've run into this while using this step on an XLIFF 2.0.

A simple null check solves this, I'll open a PR for it.