Change cudf::empty_like to not include offsets for empty strings columns

[davidwendt]

Description

Fixes cudf::empty_like to only create empty child columns for nested types. The empty child columns are needed to store the types for consistency with cudf::make_empty_column.

Closes #16490

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