title: Why does NumberFormatter's string(from:) return an optional?
body: <p><a href="https://developer.apple.com/documentation/foundation/numberformatter/1418046-string" rel="noreferrer">Documentation link</a></p>
  
  <p>Why does the <code>NumberFormatter</code> function <code>func string(from number: NSNumber) -&gt; String?</code> return a <code>String?</code> rather than a <code>String</code>? Are there particular inputs or states for a <code>NumberFormatter</code> where this function could return nil? Sometimes I hear these less-than-ideal return types are from Objective-C holdover files, is this one of those cases?</p>