Fix Issue#36

[rajakumar23]

NullSystem or NullSystemAgent should be out of for loop.

https://github.com/Apstra/apstra-api-python/blob/main/aos/devices.py#L186
https://github.com/Apstra/apstra-api-python/blob/main/aos/devices.py#L223

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[that1guy15]

Thank you! LGTM