* (bug 26861) avoid warning in OutputPage::addVaryHeader when $options param is null

* (bug 26861) avoid warning in OutputPage::addVaryHeader when $options param is null

This should coerce nulls to empty arrays to make them play a little nicer.

Also added a doc comment that the format and contents of the $option parameter need to be described in some way; judging by the below getXVO() function they're used to produce the X-Vary-Options header along with the vary cookie settings, but offhand wouldn't know what acceptable options are or how to look them up (X-Vary-Options is something we made up as an extension for our patched Squid servers, so there's no public standard like the HTTP RFCs to look it up in.)