Ruby on Rails: Using default value, when the variable is null or empty

I have this code-snippet in html erb.

For some objects the cover_image_url is empty, how do i modify this code block to use a default value ,when that property is null or empty?

<%@books.each do |book|%>
        $('#bookContainer').append('<div class="conn"><p><img class="floatright" src="<%= h book.cover_image_url%>"><h3><%= h book.title%></h3><h3><%= h book.author%></h3></p></div>');
    <% end %>