Allow nil conversion handlers in btcctl.
Allow nil conversion handlers in btcctl.

sysbot/btcd,mathiasrw/btcd,Stash-Crypto/btcd,Evil-Coin/Evil,glv2/ppcd,cqswxj2013/btcd,Dirbaio/btcd,hectorj/btcd,danoctavian/btcd,btcsuite/btcd,aakselrod/btcd,stevenroose/btcd,jcmincke/dcrd,runeaune/btcd,jimmysong/btcd,wallclockbuilder/btcd,qchain/btcd,davecgh/btcd,wallclockbuilder/btcd,jrick/dcrd,dajohi/btcd,toddfries/btcd,davecgh/btcd,ppcsuite/ppcd,dajohi/btcd,Dirbaio/btcd,FactomProject/btcd,danoctavian/btcd,cjepson/btcd,glv2/ppcd,jcmincke/dcrd,hectorj/btcd,runeaune/btcd,decred/dcrd,DanielKrawisz/btcd,mathiasrw/btcd,jrick/btcd,jcmincke/dcrd,1d4Nf6/btcd,ppcsuite/ppcd-old,pkothbauer/btcd,tb00/btcd,stevenroose/btcd,soapboxsys/ombfullnode,Dirbaio/btcd,btcsuitereleases/btcd,Roasbeef/btcd,Evil-Coin/Evil,BitGo/prova,cqswxj2013/btcd,jrick/btcd,CryptocurrencyCabal/htcd,FactomProject/btcd,Cleawing/btcd,aakselrod/btcd,dajohi/btcd,jongillham/btcd,dan-da/btcd,jimmysong/btcd,dan-da/btcd,decred/dcrd,jongillham/btcd,ppcsuite/ppcd-old,ltcsuite/ltcd,pkothbauer/btcd,sysbot/btcd,adiabat/btcd,DanielKrawisz/btcd,Cleawing/btcd,jrick/dcrd,qchain/btcd,ppcsuite/ppcd,Stash-Crypto/btcd,1d4Nf6/btcd,cjepson/btcd,btcsuitereleases/btcd,jrick/btcd,adiabat/btcd,BitGo/prova,btcsuite/btcd,tb00/btcd,ltcsuite/ltcd,toddfries/btcd,Roasbeef/btcd,CryptocurrencyCabal/htcd,soapboxsys/ombfullnode