title: Generic "is null" predicate
body: <p>I wrote this generic predicate:</p>
  
  <pre><code>private static &lt;T&gt; Predicate&lt;T&gt; isNull(){
      return Objects::isNull;
  }
  </code></pre>
  
  <p>But I can't use it in combination with other predicates like this:</p>
  
  <pre><code>private static Predicate&lt;String&gt; isEmpty(){
      return string -&gt; string.isEmpty();
  }
  </code></pre>
  
  <p>Because this snippet will not compile (expects Predicate&lt; String> in or operation):</p>
  
  <pre><code>isNull().or(isEmpty())
  </code></pre>
  
  <p>Any idea to solve it? Thanks!</p>