wallet: Make sure no DescriptorScriptPubKeyMan or WalletDescriptor members are left uninitialized after construction

Summary:

This is a small folllow-up to PR16528 ("Native Descriptor Wallets using DescriptorScriptPubKeyMan")
Before this change bool m_internal was left uninitialized when using the DescriptorScriptPubKeyMan(WalletStorage&, WalletDescriptor&) ctor.
The same goes for the now initialized integers which were left uninitialized when using the WalletDescriptor() ctor.

wallet: Make sure no DescriptorScriptPubKeyMan members are uninitialized after construction
https://github.com/bitcoin/bitcoin/pull/18782/commits/ff046aeeba8d4f3ff210d37ba020616c12450ab3

wallet: Make sure no WalletDescriptor members are uninitialized after construction
https://github.com/bitcoin/bitcoin/pull/18782/commits/2a780980983f4b4aaae75817e57e7ed308713561

This is a backport of Core PR18782

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9073