templates: disable boot OS components when no system is available

Hide the OS table and the "Boot" button when no OS is found.

ogcp/templates/actions/session.html

@@ -16,6 +16,8 @@
 
 {{ macros.cmd_selected_clients(selected_clients) }}
 
+{% if os_groups|length > 0 %}
+
 <p>
 {% if os_groups|length > 1 %}
 The selected clients have different installed OS:
@@ -51,4 +53,10 @@ The selected clients have different installed OS:
     </button>
 </form>
 
+{% else %}
+<div class="card text-center p-3">
+    <b>{{ _('No bootable OS') }}</b>
+</div>
+{% endif %}
+
 {% endblock %}