opf_manifest_epubv3.opf.erb in review-5.7.0

- old
+ new

@@ -1,7 +1,9 @@
   <manifest>
     <item properties="nav" id="<%= @config['bookname'] %>-toc.<%= @config['htmlext'] %>" href="<%= @config['bookname'] %>-toc.<%= @config['htmlext'] %>" media-type="application/xhtml+xml"/>
+<% if @config['cover'] %>
     <item id="<%= @config['bookname'] %>" href="<%= @config['cover'] %>" media-type="application/xhtml+xml"/>
+<% end %>
 <% if @coverimage %>
     <item properties="cover-image" id="cover-<%= @coverimage.id %>" href="<%= @coverimage.file %>" media-type="<%= @coverimage.media %>"/>
 <% end %>
 <% @items.sort_by { |x| x.id }.each do |item| %>
     <item id="<%= item.id %>" href="<%= item.file %>" media-type="<%= item.media %>"<%= item.properties_attribute %>/>