Fix empty md("") in Quarto

[olivroy]

Summary

Basically, the underlying problem was that charToRaw("") gives character(0).

I therefore replaced the condition !is.na(x) by nzchar(x, keepNA = FALSE) (a more elegent !is.na(x) & x != "")

I added a mocking mechanism to allow for testing in Quarto functions. I think it is worth adding to it.

For more on mocking, see https://www.tidyverse.org/blog/2023/10/testthat-3-2-0/#mocking.

I could have set a temp env var with withr::local_envvar(), but thought it read more clearly this way.

I tested my test and it indeed fails with current main!

Related GitHub Issues and PRs

Fixes #1769, amends #1690

Checklist

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

[rich-iannone]

LGTM!