fix(transport): return None from Dummy.read() when no data available

[Luminger]

Summary

Fixes 100% CPU usage when using the Dummy transport for testing.

Problem

The Dummy transport's read() method was returning bytearray(length) instead of None when no data is available. This causes the StreamDeck polling loop to never sleep, resulting in 100% CPU usage.

Solution

Changed Dummy.Device.read() to return None when no data is available, matching the behavior of LibUSBHIDAPI which returns None when hid_read() returns 0.

Verification

Run this script to verify the fix (watch CPU usage - without the fix it spins at 100%):

#!/usr/bin/env python3
import time

from StreamDeck.DeviceManager import DeviceManager

manager = DeviceManager(transport="dummy")
deck = manager.enumerate()[0]
deck.open()

start = time.time()
count = 0
while time.time() - start < 5:
    deck.read(timeout_ms=100)
    count += 1

deck.close()
print(f"Completed {count} iterations in 5 seconds")