title: mypy: Missing return statement when function does not return anything if condition failed
body: <p>I have a function that has the following type definition:</p>
  <pre><code>def func_name(kw_details: list, val_details: list) -&gt; Optional[Dict]:
      if condition:
          do something
          return dict_result
  </code></pre>
  <p>In case the condition fails, it doesn't return anything. So, I keep getting the error: <code>Missing return statement</code> from mypy.</p>
  <p>What should the typing for the function look like, in order to accommodate my logic?</p>