[chronik] Gracefully handle invalidated block with no undo data

Summary: This undo data might be missing for invalidated blocks. For now we don't bother pushing a message in this case.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D16886