Check whether $http_address$ is empty

itl/command-plugins.conf

@@ -357,6 +357,7 @@ object CheckCommand "http" {
 			description = "Host name argument for servers using host headers (virtual host)"
 		}
 		"-I" = {
+			set_if = {{ string(macro("$http_address$")) != "" }}
 			value = "$http_address$"
 			description = "IP address or name (use numeric address if possible to bypass DNS lookup)"
 		}