Return nullptr from GetLegacyScriptPubKeyMan if descriptor wallet

Summary:
Backport of Core PR16528 part [4/43] : https://github.com/bitcoin/bitcoin/pull/16528/commits/aeac157c9dc141546b45e06ba9c2e641ad86083f

Depends on D8401

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8402