Avoid treating null RPC arguments different from missing arguments

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.