SDXL ControlNet Union pipelines, make control_image argument immutible

[Teriks]

when control_image is passed a list, __call__
modifies its content, since it is pass by reference the list passed by the caller gets its content modified unexpectedly

make a copy at method intro so this does not happen

What does this PR do?

Fixes # (issue)

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[hlky]

Thanks @Teriks