wire: only write message payload if there actually is one
wire: only write message payload if there actually is one

There are certain messages that won't have a payload, e.g., verack,
causing an unnecessary write of 0 bytes.

tb00/btcd,davecgh/btcd,dajohi/btcd,btcsuite/btcd,ltcsuite/ltcd,davecgh/btcd,ltcsuite/ltcd,tb00/btcd,dajohi/btcd,dajohi/btcd,btcsuite/btcd