[Misc] fix qwen image family redundant computation (#868)

Signed-off-by: zjy0516 <riverclouds.zhu@qq.com>

examples/offline_inference/image_to_image/image_edit.py

@@ -110,7 +110,7 @@ def parse_args() -> argparse.Namespace:
     parser.add_argument(
         "--negative_prompt",
         type=str,
-        default=" ",
+        default=None,
         required=False,
     )
     parser.add_argument(

examples/offline_inference/text_to_image/text_to_image.py

@@ -24,7 +24,9 @@ def parse_args() -> argparse.Namespace:
     )
     parser.add_argument("--prompt", default="a cup of coffee on the table", help="Text prompt for image generation.")
     parser.add_argument(
-        "--negative_prompt", default="", help="negative prompt for classifier-free conditional guidance."
+        "--negative_prompt",
+        default=None,
+        help="negative prompt for classifier-free conditional guidance.",
     )
     parser.add_argument("--seed", type=int, default=142, help="Random seed for deterministic results.")
     parser.add_argument(