Treat NULL like character() in +

This is different from what's planned for `glue_data()` / `glue()` in #246, but I think it's what we want.

In any case, it preserves existing behaviour. If we want something else, it would need to happen in #246, possibly as part of an edition.

R/glue.R

@@ -354,10 +354,10 @@ as.character.glue <- function(x, ...) {
 
 #' @export
 `+.glue` <- function(e1, e2) {
-  if (!is.character(e1)) {
+  if (!is.null(e1) && !is.character(e1)) {
     stop("LHS must be a character vector.")
   }
-  if (!is.character(e2)) {
+  if (!is.null(e2) && !is.character(e2)) {
     stop("RHS must be a character vector.")
   }
 

tests/testthat/test-glue.R

@@ -523,10 +523,14 @@ test_that("`+` method does not interpolate twice", {
   expect_identical(glue("{x}", x = "{wut}") + "y", "{wut}y")
 })
 
-test_that("`+` method returns length-0 for a length-0 input", {
+test_that("`+` method returns length-0 if there is a length-0 input", {
   expect_identical(as_glue("hello") + character(), character())
 })
 
+test_that("`+` method returns length-0 if there is a `NULL` input", {
+  expect_identical(as_glue("hello") + NULL, character())
+})
+
 test_that("`+` recycles", {
   x <- c("a", "b", "c")
   expect_identical("(" + as_glue(x) + ")", paste0("(", x, ")"))