title: field validation for other field is not null
body: <p>I have 2 fields price and currency.I want to make dependency validation if price!=null currency field is required and if currency!=null price field is required as well as I want to check price field is numeric.
  if both field is null and that both field is not required.I want to validate in laravel 5.3</p>
  
  <pre><code>$this-&gt;validate($request,[
    'price'=&gt;'numeric',
    'price'=&gt;'required_if:currency,nullable',
    'currency'=&gt;'required_if:price,not nullable',
  ]);
  </code></pre>