title: Method Creating custom tokens will create uid if not exist in firebase
body: <p>I using firebase. I using method: Creating custom tokens. When i look document in firebase <a href="https://cloud.google.com/identity-platform/docs/admin/create-custom-tokens" rel="nofollow noreferrer">https://cloud.google.com/identity-platform/docs/admin/create-custom-tokens</a> .</p>
  <pre><code>const uid = 'some-uid';
  
  admin
    .auth()
    .createCustomToken(uid)
    .then((customToken) =&gt; {
      // Send token back to client
    })
    .catch((error) =&gt; {
      console.log('Error creating custom token:', error);
    });
  </code></pre>
  <p>I have a question
  If i don't have uid in firebase, it will create it ? If i have uid, i only create custom token and send back to client. I look document and i don't see document specific it. Thanks</p>