[lint] don't add newline to empty python files

Summary: It can make sense to have an empty __init__.py in a python subpackage. An empty file does not need f-strings, so return early.

Test Plan: Run arc lint --everything and check that test/functional/test_framework/__init__.py is untouched

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13597