IF you look at the source for ad_header, it shows extra_stuff_for_document_head is an optional parameter.
You can see here, page_title is required and extra_stuff_for_document_head is optional with a default value of "", an empty string. So if it is not specified in the call to ad_header, it is given a value of "".
proc_doc ad_header {page_title {extra_stuff_for_document_head ""}}
