Currently not possible to set empty a field that has a default value

Setting a 'string' property with a default value to a blank string using a 'Text' widget should set the property to a blank string and not reset it to its default value

How to reproduce:

• define a new document type with a string property and define a default value for this property
• in the edit layout, use a Text widget to edit the string property
• deploy
• create a document with the new document type and assign a value to the string property
• edit the same document and set the text property to a blank value and save the document

The text property is reset to its default value, it should contain a blank string.