<!-- Describe the issue -->

When trying to sign a transaction in a encrypted wallet that is locked, version 0.16 would return the following error:

```"Please enter the wallet passphrase with walletpassphrase first."```

While doing exactly the same procedure against 0.17, the response is something like:

```json
"hex": "020000000183ac9cc1137c444f32356fb070ecba194c87a05e43674c06a55f5b1d0561eabc0000000000ffffffff0180626a94000000001600141875aa6ab19c2551c11759cf3e9f4dd7aa40bcf500000000",
       "complete": false,
       "errors": [
         {
           "txid": "bcea61051d5b5fa5064c67435ea0874c19baec70b06f35324f447c13c19cac83",
           "vout": 0,
           "witness": [
           ],
           "scriptSig": "",
           "sequence": 4294967295,
           "error": "Unable to sign input, invalid stack size (possibly missing key)"
         }
     ]

```

<!--- What behavior did you expect? -->

v0.17 should check if the wallet is locked before trying to sign the tx and return the same error as v0.16 for consistency reasons.
{
    CPubKey vchPubKey;
    if (!pwallet->GetPubKey(keyID, vchPubKey)) {
        return;
    }
    KeyOriginInfo info;
    if (!pwallet->GetKeyOrigin(keyID, info)) {
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Internal keypath is broken");
    }
    hd_keypaths.emplace(vchPubKey, std::move(info));
}
bool FillPSBT(const CWallet* pwallet, PartiallySignedTransaction& psbtx, const CTransaction* txConst, int sighash_type, bool sign, bool bip32derivs)
{
    LOCK(pwallet->cs_wallet);
    
    bool complete = true;
    for (unsigned int i = 0; i < txConst->vin.size(); ++i) {
        const CTxIn& txin = txConst->vin[i];
        PSBTInput& input = psbtx.inputs.at(i);