title: GraphQL query returns all by default if parameter is null
body: <p>I have a simple question, I'm currently using GraphQL/Hasura to subscribe to some data, this subscription takes an id as parameter but in the case where the id is null my subcription returns all the data by default...</p>
  <p>Here is the subscription :</p>
  <pre><code>subscription MySubscription($id: Int) {
    dcim_device(where: { virtual_chassis_id: { _eq: $id } }) {
      id
      name
      serial
      site_id
      status
      ip_address {
        address
      }
      dcim_platform {
        name
      }
      device_tags {
        tag_id
        extras_tag {
          id
          name
          color
        }
      }
      dcim_devicerole {
        color
        name
      }
      virtual_chassis_id
      dcim_virtualchassis {
        id
      }
      dcim_interfaces {
        enabled
        name
        id
        type
      }
    }
  }
  </code></pre>
  <p>And here is the Vue.js front-end part in which I call my subscription with Apollo :</p>
  <pre><code>apollo: {
      $subscribe: {
        dcim_device: {
          query: require(&quot;../../graphql/subscribeToNewDevicesByVCid.gql&quot;),
          variables() {
            return {
              id: this.node.dcim_virtualchassis.id,
            };
          },
          result({ data }) {
            this.devices = data.dcim_device;
            console.log(this.devices);
          },
        },
      },
    },
  </code></pre>
  <p>As you can see &quot;node&quot; can change based on user click and there are in my case some nodes that have null virtual_chassis_id...</p>
  <p>I would like to know if it's possible to catch or something like that the case where no parameters are given to the query in order to display something else.</p>
  <p>Thanks in advance !</p>