Fix a data destruction bug when authorities are updated.

Fix a data destruction bug when authorities are updated.

Modify the two-argument form of vandelay.merge_record_xml to return
the target_marc when the add_rule, preserve_rule, replace_rule are
all empty.

This method is used by authority.propagate_changes and would replace
the bibliographic record's marc with the overlay template when the
above would happen. By returning the biblographic record entry's own
marc in that case, we prevent data destruction.

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>