wallet: let sendtoaddress skip the optional future argument cleanly (#383)

[JSanchezFDZ]

Background

Issue #383 reports that sendtoaddress "does not work except the default/simplest
send" and that anything after the future argument fails, often with a cryptic
error: Error parsing JSON:.

Testing the current code on regtest shows the feature mostly works already — the
later arguments (comment, comment_to, subtractfeefromamount, ...) are
accepted when future is given as [], null, or a valid object, and named
arguments (-named) also work. Two rough edges remain:

• Passing an empty object {} for future entered the future branch and
  failed with no future_maturity is specified.
• Passing an empty string "" for future fails at the CLI layer with
  Error parsing JSON, because future is a JSON-typed positional argument and
  "" is not valid JSON.

Change

• Treat an empty object the same as an absent future: only a non-empty
  object requests a future transaction. {}, [] and null now all mean
  "no future". A non-empty but incomplete object (e.g. only future_maturity)
  still errors exactly as before, so validation is unchanged.
• Expand the future help text to explain that the argument is optional and how
  to skip it positionally ([]/null, not "") or via -named.

Verified (regtest)

sendtoaddress <addr> 1 {} "comment" "commentto" true        -> txid (was: error -8)
sendtoaddress <addr> 1 [] "" "" true                        -> txid (unchanged)
sendtoaddress <addr> 1 {"future_maturity":50,"future_locktime":-1} "" "" true -> txid (unchanged)
sendtoaddress <addr> 1 {"future_maturity":50} "" "" true    -> error -8 "no future_locktime" (validation preserved)

Closes #383.