Handle missing columns when deleting by index

[OskarEichler]

Summary

Allow a row to return nil when the requested column index is absent, matching the existing named-column behavior. Preserve nil entries in the returned column array.

Reproduction

CSV.parse("a,b\n1,2\n", headers: true).by_col!.delete(10) raises NoMethodError because Row#delete returns nil and Table calls last on it. A shorter row can also cause an exception after earlier rows have already been changed.

Verification

10 focused checks: five failures on the release, zero with the patch, covering out-of-range positive/negative indexes, ragged rows and normal deletion.

This isolated patch passes the unchanged upstream suite in both normal and CSV_PARSER_SCANNER_TEST=yes modes: 527 tests / 4,050 assertions in each mode, zero failures/errors. Commands use Ruby 4.0.6 through rbenv (bundle exec ruby run-test.rb). Syntax and git diff --check pass. The combined release-based consumer branch also passes gem build and rake warning:error rdoc.

No tests were added or changed, per the requesting project's explicit no-new-tests policy. Focused checks and measurements live outside this repository. The upstream no-RuboCop policy is respected. Verification was local on macOS/Ruby 4.0.6; the supported OS/Ruby matrix was not run locally. Current main and installed 3.3.6 share base commit 0873ab362d996f12796c3c3e8998b5be657a9b12; no unrelated changes are included.

Breaking changes and limitations

None intended. Missing indexed columns return nil entries instead of raising an incidental NoMethodError.