Rendering comments in a Backbone view
From a once over:


load_comments should follow lowerCamelCase -> loadComments
Avoid console.log, at least wrap it in something that also takes in to account message severity and filters on that message severity
Personally I would have used either comments or wellFormedComments instead of notNullComments 
It seems that if there are no comments, that you should get out, and not do the JSON parsing and the html appending
Where is commentsTmpl defined? Smells like a global variable
It is very confusing to have #target_dom there, I am guessing that it is a subelement of #comments_section, but I would have called it #comments probably?
Even more confusing is that you append the html to #comments_section and not #target_dom


All in all, I would go with something like this:

var CommentView = Backbone.View.extend({
    el: $("#comments_section"),
    render: function() {
        var comments= comments.wellFormedComments();
        if (!comments.length) {
            return
        }
        $("#a_descriptive_id").show();
        var html = commentsTmpl(notNullComments.toJSON());
        $(this.el).append(html);
    },
});