[backport#16144] wallet: do not encrypt wallets with disabled private keys

Summary:
wallet: do not encrypt wallets with disabled private keys (whythat)

Pull request description:

Fix for #15635.
Throw an `RPC_WALLET_ENCRYPTION_FAILED` error when attempting to encrypt wallet with disabled private keys. Changed `test/function/wallet_createwallet.py` to test new behavior.

Backport of Core PR16144

Test Plan:

ninja check
test_runner.py wallet_createwallet

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7368