Fix strings::detail::copy_range when target contains nulls

[davidwendt]

Description

Fixes the logic in cudf::strings::detail::copy_range handling of nulls in the target range. The optimization check for nulls is removed simplifying the logic and making it more reliable as well. The benchmark showed no significant change in performance.
Also adds a specific gtest for this case.
Error was introduced in #15010
Closes #16618

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

/merge