Fix Vector CANlib treatment of empty app name

In Python 2, the str type was used for text and bytes, whereas in Python
3, these are separate and incompatible types. This broke instantiation
of a VectorBus when the app_name parameter in __init__ was set to None.

This correctly sets it to a bytes object.

Fixes #796

Author: karlding

can/interfaces/vector/canlib.py

@@ -112,7 +112,7 @@ def __init__(
         else:
             # Assume comma separated string of channels
             self.channels = [int(ch.strip()) for ch in channel.split(",")]
-        self._app_name = app_name.encode() if app_name is not None else ""
+        self._app_name = app_name.encode() if app_name is not None else b""
         self.channel_info = "Application %s: %s" % (
             app_name,
             ", ".join("CAN %d" % (ch + 1) for ch in self.channels),