fix: exclude none conversational fields in output

[maxduu]

https://uipath.atlassian.net/browse/JAR-9364

Before:
The conversational-agent graph would output None fields in the terminate_node, which would eventually be serialized into an empty dict (which is a separate discussion, code here and thread here).

After:
This change simply excludes the None fields from being emitted which is better in my opinion than emitting {} or null, since these fields could confuse users doing evaluations.

─────────────────────────────────────────────────────── Execution Logs: Simulated Web Search, With Chat-History ────────────────────────────────────────────────────────
  Success result output: {'uipath__agent_response_messages': [{'role': 'assistant', 'contentParts': [], 'toolCalls': [{'name': 'Web_Search', 'input': {'provider': 
'GoogleCustomSearch', 'query': 'How many people have successfully climbed Mount Everest', 'num': 10, 'start': 1}}], 'interrupts': []}, {'role': 'assistant', 
'contentParts': [{'mimeType': 'text/markdown', 'data': {'inline': 'As of 2026, it is estimated that over 6,000 people have successfully climbed Mount Everest. This 
number changes each year as new climbers reach the summit.<uip:cite title="Mount Everest Climbing Statistics 2026" 
url="https://www.example.com/everest-climbing-stats-2026"/>'}, 'citations': []}], 'toolCalls': [], 'interrupts': []}]}

Note the lack of isTranscript, name, isIncomplete