mpremote: Do not list bogus devices.

Signed-off-by: Mark Blakeney <mark.blakeney@bullet-systems.net>

tools/mpremote/mpremote/commands.py

@@ -20,6 +20,7 @@ def do_connect(state, args=None):
         if dev == "list":
             # List attached devices.
             for p in sorted(serial.tools.list_ports.comports()):
+                if p.serial_number is not None:
                     print(
                         "{} {} {:04x}:{:04x} {} {}".format(
                             p.device,