[core/lib]: remove redundant null check

[maranix]

This PR removes an unnecessary null check from copyRange method found inside list/core/list.dart.

As the leftover comment dictates that it should be removed when promotion works and references the following issue from language repo. Since the issues has been closed and the main points have been addressed and RangeError.checkValidRange method returns an int and instead throws in-case of invalid bounds.

I believe we can safely remove this check.

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