Avoid unnecessary commands in Vizio update function

[JeffLIrion]

Description:

If the power is off, we don't need to check the _volume_level, _current_input, and _available_inputs attributes; we can just set them to None.

I'm hopeful that this can help me avoid errors of the form (this occurs in pyvizio when trying to get the volume level):

[pyvizio.vizio] Failed to execute command: int() argument must be a string, a bytes-like object or a number, not 'NoneType'

Related issue (if applicable):

Pull request in home-assistant.io with documentation (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
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• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
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If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[balloob]

What do you mean with "I'm hopeful". Did you not test these changes locally?

[JeffLIrion]

I'm using this code and it works. I don't know if it fixes that error because I don't see it very often, maybe once a week on average, and I don't know what the cause is.

But whether or not it fixes that error, this is just a small PR to clean up the code -- there's no need to get further attributes when the state is off or unknown. We also don't need 3 lines to do what can be done in 1 line with a list comprehension

[JeffLIrion]

I've been using this component for a week and have yet to see that error.

[JeffLIrion]

@balloob it's been almost 3 weeks and I haven't seen that error once.