BUG: Handle IndirectObject as image filter (#2355)

Previously, we might pass "4bits" as image mode to Pillow, leading to "ValueError: unrecognized image mode".

Example: `lfilters = IndirectObject(26, 0, 139771595681120)`, whose `get_object()` would yield `['/FlateDecode']` (going into the else branch of the filter handling until now).

pypdf/filters.py

@@ -779,7 +779,7 @@ def _xobj_to_image(x_object_obj: Dict[str, Any]) -> Tuple[Optional[str], bytes,
         )
     extension = None
     alpha = None
-    filters = x_object_obj.get(SA.FILTER, [None])
+    filters = x_object_obj.get(SA.FILTER, NullObject()).get_object()
     lfilters = filters[-1] if isinstance(filters, list) else filters
     if lfilters in (FT.FLATE_DECODE, FT.RUN_LENGTH_DECODE):
         img, image_format, extension, _ = _handle_flate(