fix: only add activity output property if not none

[LeonardHd]

This PR fixes an issue where activity properties with None values were incorrectly included in the activity result dictionary, causing that no parameter not found errors are thrown when expressions tried to access non-existent properties as they are passed as None.

• Updates the expression evaluator to only include activity properties in the output body when they are not None
• Changes the default status from being automatically set to SUCCEEDED to allowing explicit None values
• Adds comprehensive test coverage for scenarios where activity properties are None

Fixes #164