Fix warning in UrlField when value is nullish and emptyText is empty

[OoDeLally]

Choosing between

• displaying a link or a static text,
• showing the value or the default,
  are two distinct orthogonal decisions.

In my base my value was undefined and I didnt provide an emptyText.
Because of that UrlField tried to create a Link leading to an annoying console warning, because it doesnt like that children is undefined.

In the case the value is null, it is better to display an empty static text than an empty link.

[fzaninotto]

Thanks!