Error earlier for empty "contents:"

[maelle]

(maybe a niche mistake) While refactoring a reference index configuration I accidentally left an empty "contents:", for which the error was not very clear.

> pkgdown::check_pkgdown()
Error in `map2()`:
ℹ In index: 38.
Caused by error in `strsplit()`:
! non-character argument
Backtrace:
  1. pkgdown::check_pkgdown()
 10. purrr::imap(., data_reference_index_rows, pkg = pkg)
 11. purrr::map2(.x, vec_index(.x), .f, ...)
 12. purrr:::map2_("list", .x, .y, .f, ..., .progress = .progress)
 16. pkgdown (local) .f(.x[[i]], .y[[i]], ...)
     ...
       at pkgdown/R/build-reference-index.R:51:4
 20. pkgdown:::ext_topics(ext_strings)
       at pkgdown/R/topics.R:186:2
 21. base::strsplit(match_strings, "::", fixed = TRUE)
       at pkgdown/R/topics-external.R:2:2

should there be some initial checks than none of contents, title, subtitle is empty in the YAML file?