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/net.cpp

@@ -193,7 +193,7 @@ UniValue getpeerinfo(const JSONRPCRequest& request)
 UniValue addnode(const JSONRPCRequest& request)
 {
     std::string strCommand;
-    if (request.params.size() == 2)
+    if (!request.params[1].isNull())
         strCommand = request.params[1].get_str();
     if (request.fHelp || request.params.size() != 2 ||
         (strCommand != "onetry" && strCommand != "add" && strCommand != "remove"))
@@ -490,7 +490,7 @@ UniValue getnetworkinfo(const JSONRPCRequest& request)
 UniValue setban(const JSONRPCRequest& request)
 {
     std::string strCommand;
-    if (request.params.size() >= 2)
+    if (!request.params[1].isNull())
         strCommand = request.params[1].get_str();
     if (request.fHelp || request.params.size() < 2 ||
         (strCommand != "add" && strCommand != "remove"))

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;

src/wallet/rpcwallet.cpp

@@ -280,7 +280,7 @@ UniValue setaccount(const JSONRPCRequest& request)
         throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
 
     std::string strAccount;
-    if (request.params.size() > 1)
+    if (!request.params[1].isNull())
         strAccount = AccountFromValue(request.params[1]);
 
     // Only add the account if the address is yours.
@@ -768,7 +768,7 @@ UniValue getbalance(const JSONRPCRequest& request)
 
     LOCK2(cs_main, pwallet->cs_wallet);
 
-    if (request.params.size() == 0)
+    if (request.params[0].isNull() && request.params[1].isNull() && request.params[2].isNull())
         return  ValueFromAmount(pwallet->GetBalance());
 
     const std::string& account_param = request.params[0].get_str();
@@ -838,11 +838,11 @@ UniValue movecmd(const JSONRPCRequest& request)
     CAmount nAmount = AmountFromValue(request.params[2]);
     if (nAmount <= 0)
         throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount for send");
-    if (request.params.size() > 3)
+    if (!request.params[3].isNull())
         // unused parameter, used to be nMinDepth, keep type-checking it though
         (void)request.params[3].get_int();
     std::string strComment;
-    if (request.params.size() > 4)
+    if (!request.params[4].isNull())
         strComment = request.params[4].get_str();
 
     if (!pwallet->AccountMove(strFrom, strTo, nAmount, strComment)) {
@@ -899,7 +899,7 @@ UniValue sendfrom(const JSONRPCRequest& request)
     if (nAmount <= 0)
         throw JSONRPCError(RPC_TYPE_ERROR, "Invalid amount for send");
     int nMinDepth = 1;
-    if (request.params.size() > 3)
+    if (!request.params[3].isNull())
         nMinDepth = request.params[3].get_int();
 
     CWalletTx wtx;
@@ -1105,7 +1105,7 @@ UniValue addmultisigaddress(const JSONRPCRequest& request)
     LOCK2(cs_main, pwallet->cs_wallet);
 
     std::string strAccount;
-    if (request.params.size() > 2)
+    if (!request.params[2].isNull())
         strAccount = AccountFromValue(request.params[2]);
 
     // Construct using pay-to-script-hash:
@@ -1711,10 +1711,10 @@ UniValue listaccounts(const JSONRPCRequest& request)
     LOCK2(cs_main, pwallet->cs_wallet);
 
     int nMinDepth = 1;
-    if (request.params.size() > 0)
+    if (!request.params[0].isNull())
         nMinDepth = request.params[0].get_int();
     isminefilter includeWatchonly = ISMINE_SPENDABLE;
-    if(request.params.size() > 1)
+    if(!request.params[1].isNull())
         if(request.params[1].get_bool())
             includeWatchonly = includeWatchonly | ISMINE_WATCH_ONLY;
 
@@ -2361,19 +2361,18 @@ UniValue lockunspent(const JSONRPCRequest& request)
 
     LOCK2(cs_main, pwallet->cs_wallet);
 
-    if (request.params.size() == 1)
-        RPCTypeCheck(request.params, {UniValue::VBOOL});
-    else
-        RPCTypeCheck(request.params, {UniValue::VBOOL, UniValue::VARR});
+    RPCTypeCheckArgument(request.params[0], UniValue::VBOOL);
 
     bool fUnlock = request.params[0].get_bool();
 
-    if (request.params.size() == 1) {
+    if (request.params[1].isNull()) {
         if (fUnlock)
             pwallet->UnlockAllCoins();
         return true;
     }
 
+    RPCTypeCheckArgument(request.params[1], UniValue::VARR);
+
     UniValue outputs = request.params[1].get_array();
     for (unsigned int idx = 0; idx < outputs.size(); idx++) {
         const UniValue& output = outputs[idx];