[FIX] website_quote: increase robustess of menu generation

On online quotations, a left menu is automatically generated on page
load by searching for the <h1/> and <h2/> elements in the page. This
also considered the empty ones which do not make sense and were
breaking the menu's layout. Ideally, there should not be any empty
title in the page.

opw-752764

addons/website_quote/static/src/js/website_quotation.js

@@ -135,12 +135,18 @@ if(!$('.o_website_quote').length) {
                     case "h1":
                         var id = self.setElementId('quote_header_', el);
                         var text = self.extractText($(el));
+                        if (!text) {
+                            break;
+                        }
                         last_li = $("<li>").append($('<a href="#'+id+'"/>').text(text)).appendTo(self.$el);
                         last_ul = false;
                         break;
                     case "h2":
                         var id = self.setElementId('quote_', el);
                         var text = self.extractText($(el));
+                        if (!text) {
+                            break;
+                        }
                         if (last_li) {
                             if (!last_ul) {
                                 last_ul = $("<ul class='nav'>").appendTo(last_li);