fix: image encoding issue when the format is not specified (#4435)

fix: give a default format when it's not specified

Signed-off-by: Frost Ming <me@frostming.com>

src/_bentoml_sdk/validators.py

@@ -76,7 +76,7 @@ def decode(self, obj: bytes | t.BinaryIO | UploadFile | PILImage.Image) -> t.Any
 
     def encode(self, obj: PILImage.Image) -> bytes:
         buffer = io.BytesIO()
-        obj.save(buffer, format=obj.format)
+        obj.save(buffer, format=obj.format or "PNG")
         return buffer.getvalue()
 
     def __get_pydantic_core_schema__(