`modify_split_caption` does not work when `tbl_listing` is empty

This renders an output with no subtitle for treatments.
```
gts_ael02 <- df_ael02 |>
crane::tbl_listing() |>
modify_split_caption(spl_col = "TRT01A", pattern = "Treatment: {spl_level}")
```

Updateing `tbl_listing` as follows `crane::tbl_listing(split_by_rows = list(variable_level = "TRT01A")) ` fixes the issue.

Reproduce the pipeline using tbl_listing() with no split_by_rows and then with variable_level set to TRT01A, focusing on how modify_split_caption() handles the empty table listing. Trace these two entry points and verify that the treatment subtitle is rendered for the first case without breaking the working case.

Written by the indexing model from the issue text.