T6523: Telegraf use nft scripts only if the firewall configured

If a firewall is not configured there is no reason to get and
execute telegraf firewall custom scripts as there are no nft
chain in the firewall nftables configuration

(cherry picked from commit ebff0c4)

data/templates/telegraf/telegraf.j2

@@ -130,7 +130,9 @@ metric_name_label = "{{ loki.metric_name_label }}"
 {% if influxdb is vyos_defined %}
 [[inputs.exec]]
   commands = [
+{%     if nft_chains is vyos_defined %}
     "{{ custom_scripts_dir }}/show_firewall_input_filter.py",
+{%     endif %}
     "{{ custom_scripts_dir }}/show_interfaces_input_filter.py",
     "{{ custom_scripts_dir }}/vyos_services_input_filter.py"
   ]

src/conf_mode/service_monitoring_telegraf.py

@@ -86,7 +86,8 @@ def get_config(config=None):
     monitoring['custom_scripts_dir'] = custom_scripts_dir
     monitoring['hostname'] = get_hostname()
     monitoring['interfaces_ethernet'] = Section.interfaces('ethernet', vlan=False)
-    monitoring['nft_chains'] = get_nft_filter_chains()
+    if conf.exists('firewall'):
+        monitoring['nft_chains'] = get_nft_filter_chains()
 
     # Redefine azure group-metrics 'single-table' and 'table-per-metric'
     if 'azure_data_explorer' in monitoring: