fix bugradio (#455)

modules/gradio_hijack.py

@@ -265,10 +265,11 @@ class Image(
         if x is None:
             return x
 
-        mask = ""
+        mask = None
+
         if self.tool == "sketch" and self.source in ["upload", "webcam"]:
-            assert isinstance(x, dict)
-            x, mask = x["image"], x["mask"]
+            if isinstance(x, dict):
+                x, mask = x["image"], x["mask"]
 
         assert isinstance(x, str)
         im = processing_utils.decode_base64_to_image(x)