[WORKFLOW SDK BUG] None Check is missing in the WorkflowState to_json

[amichel]

Expected Behavior

Always get a valid JSON using the to_json method

Actual Behavior

When workflow completes successfully, to_json (and str) methods fail with
Exception calling application: 'NoneType' object has no attribute 'message'

Steps to Reproduce the Problem

1. Run any Workflow to completion, such that failure_details is absent from the workflow state.
2. Call either get_state or wait_for_workflow_completion
3. Call to_json on the returned state object

Release Note

RELEASE NOTE:

[elena-kolevska]

LGTM. It's a simple enough change, I also confirmed it locally, but I would feel better if we added a little test for it, though.