tests: trivial simplification in pad_tx

Summary:
There is no need to initialize an empty list to just append to it unconditionaly only once.

Note that this function has a dedicated unitest, so this change is tested extensively.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11658