Bug fix - Carlini_Wagner_L2 (tf2) - raise exception when clip_min or clip_max is type None

[oiannace]

• clip_min and clip_max are used to re-scale the input data to 0-1 so they can not be None
• error currently occurs when clip_min or clip_max is None
• added code to raise exception with clear description if clip_min or clip_max is None
• removed comment saying clip_min and clip_max are optional