How to dynamically enable/disable HTTP proxy configuration based on a YAML property?

I would like to enable or disable an HTTP proxy dynamically based on a YAML property.

service:
  useHttpProxy: "true"

HTTP Request Config:<http:request-config name="service-config">
    <http:request-connection
        protocol="HTTPS"
        host="${service.host}"
        port="${service.port}"
        proxyConfig="#[
            if (Mule::p('service.useHttpProxy') == 'true')
                'proxyConfig'
            else
                null
        ]" />
</http:request-config><http:proxy
    name="proxyConfig"
    host="${proxy.host}"
    port="${proxy.port}" />

I get the following error:

Unable to convert 'proxyConfig' with class 'java.lang.String'
to class 'HttpProxyConfig'
WriterExecutionException:
Unable to convert 'proxyConfig' with class 'java.lang.String'
to class 'HttpProxyConfig'

My understanding is that the proxyConfig attribute expects an HttpProxyConfig object, while my expression returns a String ("proxyConfig"). 

Is it possible to dynamically set the proxyConfig attribute using a DataWeave expression? Because I don't like to have two separate http:request-config definitions (one with a proxy and one without) and select the appropriate one at runtime 

Thank you in advance!