Fix empty dataframe error with process_md = TRUE

[Copilot]

Summary

Fixes dplyr::recode() error when passing empty dataframes to gt() with groupname_col and process_md = TRUE.

Related GitHub Issues and PRs

• Ref: #2081

Changes

R/dt_stub_df.R

• Added guard clause to skip dplyr::recode() when unique_row_group_ids is empty (5 lines)

tests/testthat/test-gt_object.R

• Added 5 test cases covering empty/non-empty dataframes with process_md combinations

Example

# Previously threw: Error in `dplyr::recode()`: ! No replacements provided.
mtcars[0, ] |> gt(groupname_col = "cyl", process_md = TRUE)

# Now works correctly, returns empty gt_tbl

Checklist

• [x] I understand and agree to the Code of Conduct.
• [ ] I have listed any major changes in the NEWS.
• [x] I have added testthat unit tests to tests/testthat for any new functionality.

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