refactor(layout): improve margin bottom of the core block (#1116)

Improvement of commit 73af591

_layouts/page.html

@@ -5,7 +5,11 @@ layout: default
 {% include lang.html %}
 {% include origin-type.html %}
 
-<div class="row">
+{% if layout.tail_includes %}
+  {% assign has_tail = true %}
+{% endif %}
+
+<div class="row{% unless has_tail %} mb-5{% endunless %}">
   <!-- core -->
   <div id="core-wrapper" class="col-12 col-lg-11 col-xl-9 pe-xl-4">
     {% capture padding %}
@@ -52,7 +56,7 @@ layout: default
 </div>
 
 <!-- tail -->
-{% if layout.tail_includes %}
+{% if has_tail %}
   <div class="row">
     <div id="tail-wrapper" class="col-12 col-lg-11 col-xl-9 px-3 pe-xl-4 mt-5">
       {% for _include in layout.tail_includes %}