title: Collect to map skipping null key/values
body: <p>Let's say I have some stream and want to collect to map like this</p>
  
  <pre><code>stream.collect(Collectors.toMap(this::func1, this::func2));
  </code></pre>
  
  <p>But I want to skip null keys/values. Of course, I can do like this</p>
  
  <pre><code>stream.filter(t -&gt; func1(t) != null)
      .filter(t -&gt; func2(t) != null)
      .collect(Collectors.toMap(this::func1, this::func2));
  </code></pre>
  
  <p>But is there more beautiful/effective solution?</p>