<!-- 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.
const uint256& txhash = txin.prevout.hash;
        const auto it = pwallet->mapWallet.find(txhash);
        if (it != pwallet->mapWallet.end()) {
            const CWalletTx& wtx = it->second;
            CTxOut utxo = wtx.tx->vout[txin.prevout.n];
            
            input.non_witness_utxo = wtx.tx;
            input.witness_utxo = utxo;
        }
        
        if (sign && input.sighash_type > 0 && input.sighash_type != sighash_type) {
            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "Specified Sighash and sighash in PSBT do not match.");
        }
        complete &= SignPSBTInput(HidingSigningProvider(pwallet, !sign, !bip32derivs), *psbtx.tx, input, i, sighash_type);
    }
    
    for (unsigned int i = 0; i < txConst->vout.size(); ++i) {
        const CTxOut& out = txConst->vout.at(i);
        PSBTOutput& psbt_out = psbtx.outputs.at(i);
        
        SignatureData sigdata;
        psbt_out.FillSignatureData(sigdata);
        MutableTransactionSignatureCreator creator(psbtx.tx.get_ptr(), 0, out.nValue, 1);
        ProduceSignature(HidingSigningProvider(pwallet, true, !bip32derivs), creator, out.scriptPubKey, sigdata);
        psbt_out.FromSignatureData(sigdata);
    }
    return complete;
}
UniValue walletprocesspsbt(const JSONRPCRequest& request)