refactor(self-check): simplify task fallback tail

The tail of resolve_self_check_task was only reachable when task is
falsy or $-prefixed, so both branches always returned default_task.
Collapse them into a single return default_task.

Author: Pouyanpi

nemoguardrails/library/self_check/utils.py

@@ -77,10 +77,7 @@ def resolve_self_check_task(
             if event_task:
                 return event_task
 
-    if task and task.startswith("$"):
-        return default_task
-
-    return task or default_task
+    return default_task
 
 
 def get_self_check_llm(