adafruit_pio_uart.receive() should return None on no bytes received

[todbot]

To match busio.UART.receive(), adafruit_pio_uart.receive() should return None when no bytes are received.

E.g. This code below should print nothing until bytes are received. Currently it prints continuously because receive() returns an empty list.

import time
import board, busio
import adafruit_pio_uart
uart = adafruit_pio_uart.UART(tx=None, rx=board.SCL1, baudrate=31250, timeout=0.001)
while True:
    data = uart.read(3)  # read up to 3 bytes
    if data is not None:
        print("data:", ["%02x" % x for x in data])

[FoamyGuy]

Makes sense to me, the docs here confirm the behavior of returning None: https://docs.circuitpython.org/en/latest/shared-bindings/busio/index.html#busio.UART.read

Thank you!