Tolerate absence of client_id to allow edge cases

Author: rayluo

oauth2cli/oauth2.py

@@ -141,8 +141,7 @@ def __init__(
         """
         if not server_configuration:
             raise ValueError("Missing input parameter server_configuration")
-        if not client_id:
-            raise ValueError("Missing input parameter client_id")
+        # Generally we should have client_id, but we tolerate its absence
         self.configuration = server_configuration
         self.client_id = client_id
         self.client_secret = client_secret