title: Validate UTC date/date-time with ajv but allow empty strings
body: <p>I need to validate string schema format: <code>&quot;date&quot;</code> or format: <code>&quot;date-time&quot;</code> to accept only <strong>ISO 8601</strong> but also allow empty string &quot;&quot; (the &quot;&quot; requirement should be separately checked using the required schema).</p>
  <pre><code>{
     &quot;datetime1&quot;: {
        &quot;type&quot;: &quot;string&quot;,
        &quot;format&quot;: &quot;date-time&quot;,
     }
  }
  </code></pre>
  <p>However, the native ajv format parser does not allow empty string &quot;&quot; to pass. How do I make it allow empty string, and still validate ISO 8601?</p>
  <p>Do I really have to write my own format checker using <code>ajv.addFormat()</code>? I also have the same issue for <code>&quot;format&quot;: &quot;email&quot;</code>, where I also need &quot;&quot; to be valid.</p>
  <p><strong>Note:</strong> I can't modify the schema, so I can't add defaults.</p>