doc(README) Added an explanation
fix(descriptor) manage null values

null value no is described as "null" instead of "object"
in objects inside arrays, null values no longer generates mixed type for the array
fix(comparator) null values are considered of the same type as the one in the corresponding object

Signed-off-by: Gonzalo Ruiz de Villa <gonzalo.ruizdevilla@adesis.com>
test(comparator) add test for case when null property is compared to missing property

Signed-off-by: Gonzalo Ruiz de Villa <gonzalo.ruizdevilla@adesis.com>