fix: avoid None.strip() error in TCP mode by checking server_socket

[Gris333]

What problem were solved in this pull request?

Issue Number: close #xxx

Problem:
When server_socket parameter is None (which is valid when using TCP port mode only), the code calls server_socket.strip() which raises an AttributeError because None has no strip() method. This prevents the test framework from initializing properly when Unix socket is not configured.

What is changed and how it works?

Modified all occurrences of server_socket.strip() to server_socket.strip() if server_socket else '' in the following test files:

1. test/integration_test/miniob/miniob_client.py - Line 36
2. test/integration_test/miniob/miniob_server.py - Line 57
3. test/integration_test/miniob_test_util.py - Lines 129 and 391
4. test/case/miniob_test.py - Lines 113 and 277

This change ensures that when server_socket is None, it is safely converted to an empty string, allowing the code to fall back to TCP port mode. The existing logic already handles empty strings correctly by checking if len(self.__server_socket) > 0 before attempting to use Unix socket.

Other information

This is a minimal fix that improves the robustness of the test framework by handling the case where Unix socket is not configured.