Mandatory blob property element not handled correctly

A user needs to set a property of type blob as mandatory in the create layout so:

1. The user drag n drop a property of type blob in the visual editor view => Designer doesn't preview the dropzone
2. The user clicks on the element, he selects "required" => no red star displayed on the previewer but the attribute is set. Nonetheless, when the Studio project is deployed, the red star is not displayed and if the user doesn't upload the binary, the form cannot be validated but there's nothing in the UI that it is related to the dropzone.

To solve the issue, the nuxeo-dropzone needs a label value (empty by default), so that the style is correctly applied