Remove property from ARM Template when it's not needed

I'm deploying an application gateway with ARM Template and wants to loop through the creation of listeners. This is how far I got:

"copy": [
                {
                    "name": "httpListeners",
                    "count": "[length(parameters('APPLICATIONS'))]",
                    "input": {
                        "name": "[concat(parameters('APPLICATIONS')[copyIndex('httpListeners')].site,'-',parameters('APPLICATIONS')[copyIndex('httpListeners')].protocol,'listener')]",
                        "properties": {
                            "FrontendIPConfiguration": {
                                "Id": "[concat(variables('applicationGatewayID'), '/frontendIPConfigurations/', variables('frontendIpConfigName'))]"
                            },
                            "FrontendPort": {
                                "Id": "[concat(variables('applicationGatewayID'), '/frontendPorts/', variables('frontendPortName443'))]"
                            },
                            "Protocol": "[parameters('APPLICATIONS')[copyIndex('httpListeners')].protocol]",
                            "SslCertificate": {
                                "Id": "[parameters('APPLICATIONS')[copyIndex('httpListeners')].cert]"
                            },
                            "HostName": "[parameters('APPLICATIONS')[copyIndex('httpListeners')].site]",
                            "RequireServerNameIndication": "[if(equals(parameters('APPLICATIONS')[copyIndex('httpListeners')].protocol, 'HTTPS'), json('true'), json('false'))]"
                        }
                    }
                }
            ]

It works well as long as I only create HTTPS listeners, but when I create a HTTP listener I want to get rid of this part:

"SslCertificate": {
                            "Id": "[parameters('APPLICATIONS')[copyIndex('httpListeners')].cert]"
                        }

Just setting the parameter parameters('APPLICATIONS')[copyIndex('httpListeners')].cert to null doesn't help.

Any suggestions?