## 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 (!request.params[0].isNull()) {
        version = request.params[0].getInt<int>();
    }
    bilingual_str error;
    const int previous_version{pwallet->GetVersion()};
    const bool wallet_upgraded{pwallet->UpgradeWallet(version, error)};
    const int current_version{pwallet->GetVersion()};
    std::string result;
    if (wallet_upgraded) {
        if (previous_version == current_version) {
            result = "Already at latest version. Wallet version unchanged.";
        } else {
            result = strprintf("Wallet upgraded successfully from version %i to version %i.", previous_version, current_version);
        }
    }
    UniValue obj(UniValue::VOBJ);
    obj.pushKV("wallet_name", pwallet->GetName());
    obj.pushKV("previous_version", previous_version);
    obj.pushKV("current_version", current_version);
    if (!result.empty()) {
        obj.pushKV("result", result);
    } else {
        CHECK_NONFATAL(!error.empty());
        obj.pushKV("error", error.original);
    }
    return obj;
},
    };
}
RPCHelpMan simulaterawtransaction()