OAuth2/OIDC: fix Rucio Client import with auth_type oidc in 1.22.6

[gumond]

Motivation

For rucio.cfg file config:

[client]
auth_type = oidc
[...]

Doing:
from rucio.client import Client
crashes with

if self.creds['oidc_refresh_lifetime'] is None:
TypeError: 'NoneType' object is not subscriptable

Modification

rucio/client/baseclient.py

Default creds dictionary of the baseclient.py is accessible only if rucio/rucio-admin CLI is used. If Client is imported, it reads the auth_type to be oidc and looks for the self.creds dictionary which will be None in this case. The client needs to read the config from the rucio.cfg if self.creds == None.