Running master 4d661ba:
```
src/bitcoin-cli combinepsbt [""] or src/bitcoin-cli combinepsbt []
```

```
  * frame #0: 0x000000010016054a bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional_detail::optional_base<CMutableTransaction>::is_initialized(this=0x0000000000000000) const at optional.hpp:394 [opt]
    frame #1: 0x000000010016054a bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional_detail::optional_base<CMutableTransaction>::optional_base(this=<unavailable>, rhs=0x0000000000000000) at optional.hpp:196 [opt]
    frame #2: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional<CMutableTransaction>::optional(this=0x000070000b9fe7c0, (null)=0x0000000000000000) at optional.hpp:958 [opt]
    frame #3: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(PartiallySignedTransaction const&) [inlined] boost::optional<CMutableTransaction>::optional(this=0x000070000b9fe7c0, (null)=0x0000000000000000) at optional.hpp:958 [opt]
    frame #4: 0x0000000100160542 bitcoind`PartiallySignedTransaction::PartiallySignedTransaction(this=0x000070000b9fe7c0, psbt_in=0x0000000000000000) at sign.h:576 [opt]
    frame #5: 0x0000000100142ae5 bitcoind`combinepsbt(JSONRPCRequest const&) [inlined] PartiallySignedTransaction::PartiallySignedTransaction(this=0x000070000b9fe700, psbt_in=<unavailable>) at sign.h:576 [opt]
    frame #6: 0x0000000100142ae0 bitcoind`combinepsbt(request=<unavailable>) at rawtransaction.cpp:1526 [opt]
    frame #7: 0x0000000100170f0a bitcoind`CRPCTable::execute(this=<unavailable>, request=0x000070000b9fead0) const at server.cpp:557 [opt]
```
UniValue scriptsig(UniValue::VOBJ);
            scriptsig.pushKV("asm", ScriptToAsmStr(input.final_script_sig, true));
            scriptsig.pushKV("hex", HexStr(input.final_script_sig));
            in.pushKV("final_scriptSig", scriptsig);
        }
        if (!input.final_script_witness.IsNull()) {
            UniValue txinwitness(UniValue::VARR);
            for (const auto& item : input.final_script_witness.stack) {
                txinwitness.push_back(HexStr(item.begin(), item.end()));
            }
            in.pushKV("final_scriptwitness", txinwitness);
        }
        
        if (input.unknown.size() > 0) {
            UniValue unknowns(UniValue::VOBJ);
            for (auto entry : input.unknown) {
                unknowns.pushKV(HexStr(entry.first), HexStr(entry.second));
            }
            in.pushKV("unknown", unknowns);
        }
        inputs.push_back(in);
    }
    result.pushKV("inputs", inputs);
    
    CAmount output_value = 0;
    UniValue outputs(UniValue::VARR);
    for (unsigned int i = 0; i < psbtx.outputs.size(); ++i) {
        const PSBTOutput& output = psbtx.outputs[i];
        UniValue out(UniValue::VOBJ);