title: Prevent graphql query in vue-apollo if input variable is null
body: <p>I've been doing some reading and have come up with a query setup for a contact input field.  I would like to avoid running this query at component startup with null input.  I could manually run the queries through computed methods maybe, but is there a simple way to prevent this? </p>
  
  <pre><code>apollo: {
      search: { 
          query: () =&gt; contactSearchGQL,
          variables() { 
              return { 
                  searchText: this.searchText, 
              };
          },
          debounce: 300,
          update(data) { 
              console.log("received data: " + JSON.stringify(data));
          },
          result({ data, loading, networkStatus }) {
              console.log("We got some result!")
          },
          error(error) {
              console.error('We\'ve got an error!', error)
          },
          prefetch() { 
              console.log("contact search, in prefetch");
              if ( this.searchText == null ) return false;
              return true;
          },
      },
  },
  </code></pre>
  
  <p>I think I'm not understanding something about prefetch, or if it's even applicable here?  </p>