Cleanup some Json::Value methods:

* Rename isArray to isArrayOrNull
* Rename isObject to isObjectOrNull
* Introduce isArray and isObject
* Change as many uses of isArrayorNull to isArray as possible
* Change as many uses of isObjectorNull to isObject as possible
* Reject null JSON arrays for subscribe and unsubscribe

src/ripple/rpc/handlers/GatewayBalances.cpp

@@ -116,7 +116,8 @@ Json::Value doGatewayBalances (RPC::Context& context)
         Json::Value const& hw = params[jss::hotwallet];
         bool valid = true;
 
-        if (hw.isArray())
+        // null is treated as a valid 0-sized array of hotwallet
+        if (hw.isArrayOrNull())
         {
             for (unsigned i = 0; i < hw.size(); ++i)
                 valid &= addHotWallet (hw[i]);