fix: image conversion for other image modes (skip alpha band) (#712)

pillow image modes: https://pillow.readthedocs.io/en/stable/handbook/concepts.html

In my case, i was dealing with a `P` mode image.  and got hit with `IndexError: tuple index out of range`. While the current operation is useful it's not needed for cases such as the one I faced. So we put it under a condition.

PyPDFForm/image.py

@@ -42,8 +42,11 @@ def any_image_to_jpg(image_stream: bytes) -> bytes:
         return image_stream
 
     rgb_image = Image.new("RGB", image.size, (255, 255, 255))
-    rgb_image.paste(image, mask=image.split()[3])
-
+    if len(image.split()) == 4:
+      rgb_image.paste(image, mask=image.split()[3])
+    else:
+      rgb_image.paste(image)
+
     with BytesIO() as _file:
         rgb_image.save(_file, format="JPEG")
         _file.seek(0)