title: Negation of filterWhen and hasElement
body: <p>I'm learning Flux and Mono and am hitting a wall when trying to check if a key doesn't exist in redis. I need to filter my flux and want to remove the element if the key exists. I don't find a way to do this in the docs, all I can filter is to keep if the key exists. I need the opposite. </p>
  
  <pre><code>client.request(MWS_EU, Orders, ordersRequest, ListOrdersResponse.class)
      .flatMapIterable(listOrdersResponse -&gt; listOrdersResponse.getOrders())
      .filterWhen(order -&gt; isNewOrder(order.getOrderId()))
      .flatMap(...)
  </code></pre>
  
  <p>and the check for redis:</p>
  
  <pre><code>private Mono&lt;Boolean&gt; isNewOrder(String orderId) {
      return redisOrders.opsForValue().get(orderId).hasElement();
  } 
  </code></pre>
  
  <p>What is a clean way to filter my Flux to keep elements only if the key does not exist?</p>
  
  <p>I don't want to use <code>block()</code>.</p>