Don't cancel non-existant task

[AlexanderGrooff]

When HA fails to start (for whatever reason), no task is created. However, it still triggers the entry.async_on_unload callback where it tries to cancel this task:

2024-05-14 05:53:12.347 ERROR (MainThread) [homeassistant] Error doing job: Task exception was never retrieved
Traceback (most recent call last):
  File "/config/custom_components/enphase_envoy/__init__.py", line 240, in _async_stop
    await _cancel_realtime_task(task)
  File "/config/custom_components/enphase_envoy/__init__.py", line 255, in _cancel_realtime_task
    task.cancel()
    ^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'cancel'

I've added type annotations to make the problem clearer.

[sonarcloud[bot]]

Quality Gate passed

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[vincentwolsink]

Hey @AlexanderGrooff! Thanks 👍