title: Laravel 5.4 - exists validation but not required
body: <p>I have an 'array type' dropdown field in my form, e.g:</p>
  
  <pre><code>&lt;select name="category_id[]"&gt;
      &lt;option value=""&gt;Please Select&lt;/option&gt;
      // more options
  &lt;/select&gt;
  </code></pre>
  
  <p>There are 3 of these same fields (hence it being an array type) and they are all optional, but if a value is selected it checks whether it is a valid value as follows:</p>
  
  <pre><code>$rules['category_id'] = 'exists:universities,id';
  </code></pre>
  
  <p>The problem I'm having is that if the empty option is selected, it is still giving me a validation error, e.g "The selected category is invalid." If I select a valid value I don't get any errors (as expected).</p>
  
  <p>I have tried adding both <code>nullable</code> and <code>sometimes</code> to the validation rule but they don't make any difference. Do I need to do something different with it being an array type field?</p>