[client]: unify empty state handling

[pdamoune]

Proposed changes

Original issue :

{
  "timestamp": "2025-08-12T14:52:15.286095Z", 
  "level": "ERROR", 
  "name": "Microsoft Sentinel", 
  "message": "Error in ListenStream loop, exit.", 
  "exc_info": "Traceback (most recent call last):
File \"/usr/local/lib/python3.12/site-packages/pycti/connector/opencti_connector_helper.py\", line 786, in run
    state[\"start_from\"] = str(msg.id)
    ~~~~^^^^^^^^^^^
TypeError: 'NoneType' object does not support item assignment", 
"attributes": {"reason": "'NoneType' object does not support item assignment"}}

In order to handle the reset of the state in the UI for stream connectors, this conditions had been implemented in the past :
38fc5c4#diff-d5f76d595ba569b42f34c9cbf943833f0e132010315ed4680b7966f2ee961868R418-R422

+                        if state is None:
+                            self.exit = True
+                        else:
+                            state["start_from"] = str(msg.id)
+                            self.helper.set_state(state)

However, no else condition was added here :
38fc5c4#diff-d5f76d595ba569b42f34c9cbf943833f0e132010315ed4680b7966f2ee961868R428-R429

+                        if state is None:
+                            self.exit = True

Also, an error in the code had been fixed by this code :
2efa061#diff-d5f76d595ba569b42f34c9cbf943833f0e132010315ed4680b7966f2ee961868L519

-                            self.exit = True
+                            self.exit_event.set()

And wasn't changed on L786

This Fix handle the NoneType error and fix the exit_event.set()

Related issues

• [Microsoft Sentinel Intel] state[\"start_from\"] error from new Microsoft sentinel intel connector connectors#4482

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[helene-nguyen]

@pdamoune Thank you for this fix !

Tested and all good for me :)