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

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk bugfix that only changes cleanup behavior to avoid calling `.close()` on a `None` session when connection pooling is disabled.
> 
> **Overview**
> Prevents a potential `AttributeError` during request cleanup by updating `ClientBase._close()` to only close the session when connection pooling is disabled *and* `self._current_session` is not `None`.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit b1d40c54f57d0a8706957f970cf6d67db4f025b2. Bugbot is set up for automated code reviews on this repo. Configure [here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->