title: Unable to use type string as sql.NullString
body: <p>I am creating a <code>gorm</code> model</p>
  
  <pre class="lang-golang prettyprint-override"><code>// Day is a corresponding day entry
  type Day struct {
      gorm.Model
      Dateday   string         `json:"dateday" gorm:"type:date;NOT NULL"`
      Nameday   string         `json:"nameday" gorm:"type:varchar(100);NOT NULL"`
      Something sql.NullString `json:"salad"`
      Holyday   bool           `json:"holyday"`
  }
  </code></pre>
  
  <p>I am using <code>sql.NullString</code> for the field <code>Something</code> cause it may be NULL.</p>
  
  <p>So when I try to execute a typical <code>gorm</code> example to validate my setup works:</p>
  
  <pre><code>    db.Create(&amp;Day{
          Nameday:     "Monday",
          Dateday:     "23-10-2019",
          Something:   "a string goes here",
          Holyday:      false,
      })
  </code></pre>
  
  <p>I get:</p>
  
  <blockquote>
    <p>cannot use "a string goes here", (type string) as type sql.NullString in field value</p>
  </blockquote>
  
  <p>What type should I use for the <code>Something</code> field given it may be NULL?</p>