validation: remove redundant check on pindex

Summary:
This removes a conditional that checks if pindex is equal to nullptr.
This check is redundant because the branch where pindex is set returns at an earlier time. Additionaly, The independence of the earlier and later pindex is made clearer.

This is a backport of core#20868

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11772