Handle empty bytes in echo

[Iain-S]

Previously, an empty byte string could cause an exception if nl=True.

This PR narrows the type scope of out, to bytes() | bytearray() | str and handles all cases exhaustively.

We also change from typing.Any, which effectively disables typing for a variable, to object, which is more restrictive. See https://mypy.readthedocs.io/en/latest/dynamic_typing.html#any-vs-object for more.

fixes #3487