Fix RevDel_RevisionItem::getAuthorNameField to work for ips

Fix RevDel_RevisionItem::getAuthorNameField to work for ips

The user_name column in the database result is null for anon users,
that will always return null when calling getAuthorName.

The function getAuthorName is used when the log_search table is filled
for revision deletion. After this fix the value for 'target_author_ip'
will be there which allows searching for logs where edits of the ip was
suppressed (because the search field 'offender' only works for oversighter)

Change-Id: I611bc56eefac594a28ad4808fe7d9ba48c872922