title: spring mock mvc: ArgumentMatcher returning null
body: <p>Why is ArgumentMatcher &quot;eq&quot; returning null?</p>
  <pre><code>List&lt;String&gt; external = List.of(&quot;abc&quot;)
  Matchers.contains(eq(external.get(0)) // ==&gt; eq(external.get(0)) returns null
  Matchers.contains(external.get(0))     // ==&gt; This returns &quot;abc&quot;
  </code></pre>