wg.conf: do not output list keys if they are a string

wireguard/files/wg.conf

 	{%- endif -%}
 {%- endmacro -%}
 {%- macro output_list_if_set(object, lookup_key, output_key) -%}
-	{%- for item in object.get(lookup_key, []) -%}
+	{%- set l = object.get(lookup_key, []) -%}
+	{%- if l is not string -%}
+		{%- for item in l -%}
 {{output_key}} = {{item}}
-	{%- endfor -%}
+		{%- endfor -%}
+	{%- endif -%}
 {%- endmacro -%}
 [Interface]
 {{ output_list_if_set(interface, 'address', 'Address') }}