title: Laravel validation rules if field empty another field required
body: <p>I have a Laravel 5.3 app, and an update form where I send user profile values. One of them is user image as well, I am creating a hidden input field for it, so if the user is not uploading any new image, I can still see that he has the hidden field with the old image value. </p>
  
  <pre><code>&lt;input type="hidden" class="form-control" name="old_image" value="{{ $player-&gt;image_filename }}" id="oldImage"&gt;
  </code></pre>
  
  <p>If he removes the image, the hidden input value becomes empty. </p>
  
  <pre><code>document.getElementById('oldImage').value = '';
  </code></pre>
  
  <p>That is how I thought of updating a user image. But I don't know how to set up validation rules for that form. Something that would be similar to this:</p>
  
  <pre><code>    public function rules()
      {
          return [
            'first_name' =&gt; 'required|max:50',
            'last_name' =&gt; 'required|max:50',
            'birthday' =&gt; 'required',
            'image' =&gt; required if old_image empty
          ];
      }
  </code></pre>
  
  <p>I have tried with <code>'image' =&gt; 'required_without:old_image',</code> and also with <code>'image' =&gt; 'required_if:old_image, null',</code> but none of them showed any error message for missing image. I am showing error messages like this:</p>