scripted-diff: Rename ValidAsCString to ContainsNoNUL

Summary:

-BEGIN VERIFY SCRIPT-
 sed -i 's,ValidAsCString,ContainsNoNUL,g' $(git grep -l ValidAsCString)
-END VERIFY SCRIPT-

This concludes backport of core#25001
https://github.com/bitcoin/bitcoin/pull/25001/commits/fa7078d84fc2858a466bc1a85404f821df682538

Depends on D17151

Test Plan: ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17152