Fix null check on failed trace generation in preview mode

generate_holistic_traces.py

@@ -349,8 +349,11 @@ def main():
             print(f"  Profile: {json.dumps(ex['profile'], indent=4)}", flush=True)
             
             trace = generate_single_trace(client, deployment, ex["comments"], ex["profile"])
-            print(f"  Trace ({len(trace.split())} words):", flush=True)
-            print(f"    {trace}", flush=True)
+            if trace:
+                print(f"  Trace ({len(trace.split())} words):", flush=True)
+                print(f"    {trace}", flush=True)
+            else:
+                print(f"  ⚠ FAILED to generate trace", flush=True)
         
         print(f"\n=== Preview complete. Run without --preview to generate all traces. ===", flush=True)
         return