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.