return null when totalBytesToSendMax cannot be evaluated (instead of 0)

lib/server.js

@@ -981,7 +981,7 @@ WalletService.prototype.getBalance = function(opts, cb) {
       if (err) {
         log.error('Could not compute size of send max transaction', err);
       }
-      balance.totalBytesToSendMax = size || 0;
+      balance.totalBytesToSendMax = size || null;
       return cb(null, balance);
     });
   });