Add explicit handling for length 0 variables in glue

[PettyCode]

Currently, the glue function returns character(0) when one of the variables to be evaluated has length 0 and is not NULL. The NULL case is already handled by the .null parameter. However, the behaviour with length 0 objects can be confusing and potentially lead to unintended consequences.

I propose adding a new feature that properly handles length 0 variables in the glue function. This feature should provide a clear and consistent behaviour for dealing with length 0 objects, making it easier for users to understand and work with the output in such cases.

I have already implemented a working solution with test cases using an additional parameter, analogous to .null. I would like to contribute it to the tidyverse/glue package. Before creating a pull request, I wanted to open an issue to discuss the potential changes, gather feedback, and ensure that the new feature aligns with the project's goals and guidelines.

Please let me know your thoughts on this proposed feature, and if it's something you'd be interested in incorporating into the tidyverse/glue package.

Current implementation:

n1 <- "foo"
n2 <- character()
res <- glue::glue("{n1} and {n2} go together")
print(res)
#Output: character(0)

Suggested implementation:

n1 <- "foo"
n2 <- character()
res <- glue::glue("{n1} and {n2} go together", .empty = "bar")
print(res)
#Output: "foo and bar go together"

[jennybc]

Yes, behaviour for length 0 input needs attention.

However, I'm closing this issue and the associated PR, because #246 already captures the need to better align glue with tidyverse recycling rules.