title: GraphQL disable filtering if filter variable is empty
body: <p>I have a Gatsby GraphQL query for a list of posts ordered by date and filtered by category.</p>
  
  <pre><code>{
    posts: allContentfulPost(
      sort: {fields: [date], order: DESC},
      filter: {category: {slug: {eq: $slug}}}
    ) {
      edges {
        node {
          title {
            title
          }
          date
        }
      }
    }
  }
  </code></pre>
  
  <p>Right now when <code>$slug</code> is the empty string <code>""</code>, I get</p>
  
  <pre><code>{
    "data": {
      "posts": null
    }
  }
  </code></pre>
  
  <p>Is there a way to get all posts instead?</p>