Only add suffix if it is not set to false

Removing the urlSuffix in Page.php by setting it to false works in source, but javascript adds it again. Resulting in link with undefined at the end.

**Setting URLSuffix to false, if page is published:**
```
public function getCMSFields()
{
      $fields = parent::getCMSFields();
        if( $this->isPublished() ) {
            $fields->FieldByName('Root.Main.URLSegment')->setURLSuffix(false);
        }
}
```
See issue: silverstripe#2723
silverstripe#2723

client/src/legacy/SiteTreeURLSegmentField.js

@@ -34,7 +34,7 @@ $.entwine('ss', function($) {
 			}
 
 			// Transfer current value to holder
-			this.find('.URL-link').attr('href', encodeURI(url + field.data('suffix'))).text(previewUrl);
+      this.find(".URL-link").attr("href", encodeURI(url + (field.data('suffix')?field.data('suffix'):''))).text(previewUrl);
 		},
 
 		/**