ConfigSetAdminRequest.CREATE should allow null in baseConfigSetName

Currently,
ConfigSetAdminRequest.Create gives an exception no Base ConfigSet specified! if baseConfigSetName is null.

However, a configSet can be created by passing the _default as the baseConfigSetName which is a hack.

IMO baseConfigSetName should be optional, so that, instead of giving an exception, ConfigSetAdminRequest.Create lets the user create a configSet from the default config set i.e. _default if the baseConfigSetName is not provided.