Summary

• Expose ClientConfig.update_cache_when_empty through ClientConfigBuilder.update_cache_when_empty(...).
• Keep the existing default behavior unchanged.
• Add unit tests for the default value, configured value, and builder chaining.

Test plan

• ./.venv/bin/python -m unittest test.test_client_config_builder -v
• ./.venv/bin/python -m compileall v2/nacos/common/client_config_builder.py test/test_client_config_builder.py
• ./.venv/bin/python -m unittest test.test_client_config_builder test.test_naming_service_cache -v
• Local Nacos smoke test: default update_cache_when_empty=False does not receive the empty callback after the last instance is deregistered; .update_cache_when_empty(True) receives [].

Related to alibaba/nacos#15419