JsonFragment: get rid of EMPTY

It is a hindrance for what comes next. And we get rid of a "reference equals".

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
ValidationReport: change .toString() to append the current schema context

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
SchemaNode: improve .toString()

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>