fix: guard None client_kwargs when unpacking for s3 client

mypy 2.1.0 flags `**client_kwargs` where client_kwargs is dict | None
(factory.py:203). It's also a latent runtime crash: version_aware=True with
client_kwargs=None would raise TypeError. Default to an empty dict.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: shcheklein

src/pytest_servers/factory.py

@@ -200,7 +200,7 @@ def s3(
             from botocore.session import Session
 
             session = Session()
-            client = session.create_client("s3", **client_kwargs)
+            client = session.create_client("s3", **(client_kwargs or {}))
             client.create_bucket(
                 Bucket=bucket_name,
                 ACL="public-read",