Use ToHex() in feature_nulldummy.py

Summary:
Avoid using the long form bytes_to_hex_str(obj.serialize()) and makes
the code more readable.

Test Plan:

./test/functional/test_runner.py feature_nulldummy

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2537