Generate a warning (or other condition) for empty @examplesIf ? @examples?

[MichaelChirico]

Recently filed vincentarelbundock/marginaleffects#1338 where @examplesIf was used slightly incorrectly:

#' @examplesIf interactive() || isTRUE(Sys.getenv("R_DOC_BUILD") == "true")
#' @examples

I think there was an author reasoning mistake about how @examplesIf is meant to work, like "write the condition in @examplesIf, then the examples to be conditioned below under @examples.

This mistake could easily go un-noticed, and I can't easily see a reason to have an empty @examplesIf entry, so it might make sense to warn in this case (with a roxy-classed condition, of course).

More generally, I also don't see why you'd have an empty @examples section either (besides as a placeholder), so a warning could be appropriate there also.