Handle an Exception without a traceback.

When an exception does not have a traceback there are no frames
to filter, but there's an explicit check for the last frame which
will fail if there are no frames.

An example exception with this problem can be seen when using
concurrent.futures.ProcessPoolExecutor.  When it reconstitutes a
remote exception it has serialized the traceback to a string it
includes as the error message instead of an object that can be
inspected.

src/werkzeug/debug/tbtools.py

@@ -357,6 +357,10 @@ def __init__(
             tb = tb.tb_next  # type: ignore
 
     def filter_hidden_frames(self) -> None:
+        # If we don't have any frames to start with, there's nothing to filter
+        if not self.frames:
+            return
+
         new_frames: t.List[Frame] = []
         hidden = False