Audited where named args will fail to use correct default values or may fail when additional optional arguments are added.

Previously for these parameters, it was fine to omit them as positional arguments, but it would trigger UniValue runtime errors to set them to null, or to omit them while passing named parameters with greater positions (which would internally set earlier missing arguments to null). Now null values are treated the same as missing values so these errors do not occur.

Included a few other small fixes while working on it.

I didn't bother fixing account-based rpc calls.