wallet: Return info for unmined tickets
wallet: Return info for unmined tickets

This fixes the GetTicketInfo/GetTicketInfoPrecise wallet functions to
return an empty block header for unmined tickets, instead of failing.

This makes these calls behave the same way as the GetTickets call.

jrick/dcrwallet,jrick/dcrwallet,jrick/btcwallet,jrick/btcwallet,decred/dcrwallet,decred/dcrwallet