Null JSON values are parsed as "null" strings

[mickael-menu]

When a RWPM contains a null value, it is interpreted as the "null" string in the parsed model. For example:

{
  "href": "chap.xhtml",
  "type": "application/xhtml+xml",
  "title": null
}

[mickael-menu]

It's actually a "bug" in Java APIs: https://stackoverflow.com/questions/18226288/json-jsonobject-optstring-returns-string-null

But I think we can tackle this in our custom optNullableString().