fix: use all Heatmap samples when selection blank

[phoman14]

Changes

• Treat a blank sample.names selection in heatmapSC() as all values in the selected sample metadata column.
• Document the blank-selection behavior in roxygen comments.
• Add a focused regression test.
• Add a NEWS entry for the user-facing behavior change.

Issues

None.

PR Checklist

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Write unit tests for any new features, bug fixes, or other code changes.
• Update the docs if there are any API changes (roxygen2 comments, vignettes, readme, etc.).
• Update NEWS.md with a short description of any user-facing changes and reference the PR number.
• Run devtools::check() locally and fix all notes, warnings, and errors. Not run per requested focused-test scope; devtools::load_all() followed by tests/testthat/test-Heatmap.R passed locally (47 passes, 7 expected CRAN skips).