Fix for type error caused by appending byte arrays to empty string

[connormbrooks]

Fixes a TypeError caused when clients send incoming message as byte arrays. This error is introduced in ROS 2 due to Python 3's introduction of byte arrays distinct from strings.

[mvollrath]

It looks like if self.buffer is a non-zero-length str and a bytearray comes in it will raise a TypeError then continue to raise more TypeError for each bytearray until another str comes in to clear self.buffer.

[connormbrooks]

Yes - this still retains the error if byte arrays and strings are intermixed, although it fixes the issue if a client is only sending one or the other.

[mvollrath]

Thank you 👍