Compress Slate values before saving

[tiberiuichim]

I propose that we do a denormalization on save and a normalization on "load" of values. The normalization algoritm is the one in this function:

volto-slate/src/widgets/HtmlSlateWidget.jsx

Line 23 in 0a8f5fd

const normalizeToSlate = (editor, nodes) => {

The denormalization algorithm would be the "reverse" algorithm, it would remove the empty {text: ''} placeholders so that the JSON value is compressed before save. These placeholder nodes only make sense for SlateJS internals, they shouldn't have been exposed "to the world".