What does this question mark mean in Flow: "?() => void"
<p>In a <a href="https://github.com/fbsamples/f8app/blob/b5df451259897d1838933f01ad4596784325c2ad/js/store/configureStore.js#L47-L47" rel="noreferrer">GitHub project</a> I recently saw this function declaration:</p>

<pre><code>function configureStore(onComplete: ?() =&gt; void) {
</code></pre>

<p>What this question mark is about?</p>

<p>I guess, <code>onComplete</code> is named parameter, getting function calls. And the question mark is stating that this parameter could be optional and will default to "void", which would mean the same like a nil/null pointer what means "no closure" assigned here.</p>

<p>Am I right?</p>