Avoid trying to set values that are null

templates/etc/nagios/nrpe.cfg.j2

@@ -4,7 +4,7 @@
 {%- if (nagios_nrpe_agent_config[k] is iterable) and
        (nagios_nrpe_agent_config[k] is not string) %}
 {{ k }}={{ nagios_nrpe_agent_config[k] | join(', ') }}
-{%- else %}
+{%- elif nagios_nrpe_agent_config[k] is not none %}
 {{ k }}={{ nagios_nrpe_agent_config[k] }}
 {%- endif %}
 {% endfor %}