Fix potential AttributeError in session cleanup

[bpeterman]

Problem

File "customerio/client_base.py", line 112, in _close

...
AttributeError: 'NoneType' object has no attribute 'close'

The _close() method in ClientBase was attempting to call .close() on self._current_session without checking if it was None first. This could lead to an AttributeError when:

• Connection pooling is disabled (use_connection_pooling=False)
• The session hasn't been initialized yet or has been set to None

Solution

Added a null check to the condition in _close() method to ensure we only attempt to close the session if it actually exists:

# Before
if (not self.use_connection_pooling):
    self._current_session.close()  # Could raise AttributeError if None

# After  
if (not self.use_connection_pooling and self._current_session is not None):
    self._current_session.close()  # Safe - only called when session exists