Fix segmentation fault in empty repository

When a repository is empty, the ATOM feed link is written in the header,
but this involves formatting ctx->qry.head which is NULL in this case.
With glibc, vsnprintf formats "%s" with a NULL input as "(null)" but on
Solaris this results in a segmentation fault.  Since we don't have a
meaningful head for the atom feed in an empty repository, it's simplest
not to write out the link element at all.

Signed-off-by: John Keeping <john@metanate.com>