Avoid treating null RPC arguments different from missing arguments

This changes RPC methods to treat null arguments the same as missing arguments,
instead of throwing type errors. Specifically:

- `getbalance` method now returns the wallet balance when the `account` param
  is null instead of throwing a type error (same as when parameter is missing).
  It is still an error to supply `minconf` or `watchonly` options when the
  account is null.

- `addnode` and `setban` methods now return help text instead of type errors if
  `command` params are null (same as when params are missing).

- `sendrawtransaction`, `setaccount`, `movecmd`, `sendfrom`,
  `addmultisigaddress`, `listaccounts`, `lockunspent` methods accept null
  default values where missing values were previously allowed, and treat them
  the same.

src/rpc/rawtransaction.cpp

@@ -922,7 +922,7 @@ UniValue sendrawtransaction(const JSONRPCRequest& request)
     const uint256& hashTx = tx->GetHash();
 
     CAmount nMaxRawTxFee = maxTxFee;
-    if (request.params.size() > 1 && request.params[1].get_bool())
+    if (!request.params[1].isNull() && request.params[1].get_bool())
         nMaxRawTxFee = 0;
 
     CCoinsViewCache &view = *pcoinsTip;