title: Validate a Go struct field only if it is populated
body: <p>I'm using this library to validate my Go struct.
  <a href="https://pkg.go.dev/github.com/go-playground/validator/v10" rel="nofollow noreferrer">https://pkg.go.dev/github.com/go-playground/validator/v10</a></p>
  <p>How do I validate a field only if it's populated? For example, I have an optional phone number field in one of my structs. If the user has provided this value, I would like to validate it using E.164 format.</p>
  <pre><code>Phone string  `validate:&quot;e164&quot;`
  </code></pre>
  <p>I searched for &quot;optional&quot; tag but couldn't find one.</p>