DataMapper not changing the value when given null on a regex expression

[YoussefDahi]

Description:
I've noticed that in the map object that we pass in the transformer of a DataMapper. When we have a regex and we pass a value of "null", DataMapper detects the regex but do not change the old value to null.

Step to Reproduce:

1. in transformer.json :
   { "map": { "(\\w*)_code": null } }, "@type": "DATA_MAPPER" }

2. in the tests.json we need to have :
   [ { "value": userName, "expected": null } ]

Expected behavior:

The 'userName' should be changed to 'null'

Actual behavior:

The 'userName' remains the same