fix: Add null check for logic node expression to prevent AttributeError

Problem: Logic nodes without logic_expression defined caused AttributeError
"'NoneType' object has no attribute 'operator'" when evaluating condition.

Solution: Check both node.condition AND node.condition.expression before
calling evaluate_logic_expression(). Return clear FAILED state with error
message instead of crashing.

Impact: Workflows with incomplete logic node definitions now fail gracefully
with clear error message instead of cryptic AttributeError.

backend/workflow_executor.py

@@ -398,8 +398,16 @@ def execute_logic_node(
     started_at = datetime.utcnow().isoformat()
 
     try:
-        if not node.condition:
+        if not node.condition or not node.condition.expression:
-            raise ValueError(f"Logic node {node.id} has no condition")
+            error_msg = f"Logic node {node.id} has no condition/expression defined"
+            logger.error(error_msg)
+            return NodeExecutionState(
+                node_id=node.id,
+                status=NodeStatus.FAILED,
+                error=error_msg,
+                started_at=started_at,
+                completed_at=datetime.utcnow().isoformat()
+            )
 
         # 1. Evaluiere Bedingung
         result, error = evaluate_logic_expression(node.condition.expression, context)