Fixes #16624: Set allow_null=True on method fields that can return None

[fabi125]

Fixes: #16624

Noticed that Device.parent_device was not marked as nullable in the swagger api docs. Went through all uses of SerializerMethodField and added allow_null=True if the method could return None.

I opted to add it to the @extend_schema_field decorator where possible because that's how allow_null was set on other methods that could return None.

[github-actions[bot]]

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