## Problem

Encrypting a wallet through JSON RPC or Qt *appears to* allow a user to include null characters in the passphrase, but silently ignores everything including and after the first null character.

For instance (on regtest), trying to set a passphrase of "`a{null character}b`":

`curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "encryptwallet", "params": ["a\u0000b"]}' -H 'content-type: text/plain;' http://127.0.0.1:18443/`

This will succeed, but allow the user to unlock with the passphrase "`a`", instead of the expected full passphrase (which also works):

`curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "walletpassphrase", "params": ["a",10]}' -H 'content-type: text/plain;' http://127.0.0.1:18443/`

I am also able to replicate it in Qt on my macOS machine by running `printf 'a\0b' | pbcopy` and pasting the result into the passphrase dialog. 

My main concern is a user thinking that they're generating, say, 32 random bytes as a passphrase, and if they're unlucky and get a zero in the first few bytes, it unexpectedly cuts their security down to almost nothing. 

## Root Cause

The reason is due to our `SecureString` type. `SecureString` is a `std::string` specialization with a secure allocator. However, when assigned, it's treated like a C- string (no explicit length and null-terminated). See the [original PR](https://github.com/bitcoin/bitcoin/pull/666) for more details.

## Potential Solutions

I think there are two plausible approaches to take. The first (and my preference) is to allow and support null characters, and I will submit a PR that enables that (by making `SecureString` use the entire string). The second is to explicitly reject any passphrases that contain null characters.

One significant complication may be that, if anyone is *already* using a passphrase with a null, then my first solution would stop their wallet from unlocking. However, it would still be unlockable just by trimming the null and any subsequent characters.
if (spk_man) {
        CKeyID seed_id = spk_man->GetHDChain().seed_id;
        if (!seed_id.IsNull()) {
            obj.pushKV("hdseedid", seed_id.GetHex());
        }
    }
    if (pwallet->CanSupportFeature(FEATURE_HD_SPLIT)) {
        obj.pushKV("keypoolsize_hd_internal",   (int64_t)(pwallet->GetKeyPoolSize() - kpExternalSize));
    }
    if (pwallet->IsCrypted()) {
        obj.pushKV("unlocked_until", pwallet->nRelockTime);
    }
    obj.pushKV("paytxfee", ValueFromAmount(pwallet->m_pay_tx_fee.GetFeePerK()));
    obj.pushKV("private_keys_enabled", !pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS));
    obj.pushKV("avoid_reuse", pwallet->IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE));
    if (pwallet->IsScanning()) {
        UniValue scanning(UniValue::VOBJ);
        scanning.pushKV("duration", pwallet->ScanningDuration() / 1000);
        scanning.pushKV("progress", pwallet->ScanningProgress());
        obj.pushKV("scanning", scanning);
    } else {
        obj.pushKV("scanning", false);
    }
    obj.pushKV("descriptors", pwallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS));
    obj.pushKV("external_signer", pwallet->IsWalletFlagSet(WALLET_FLAG_EXTERNAL_SIGNER));
    return obj;
},
    };
}
static RPCHelpMan listwalletdir()