tests: add edge-case tests for str_flatten() and str_flatten_comma()

tests/testthat/test-flatten.R

@@ -24,3 +24,43 @@ test_that("str_flatten_oxford removes comma iif necessary", {
   expect_equal(str_flatten_comma(letters[1:3], " or "), "a, b or c")
   expect_equal(str_flatten_comma(letters[1:3]), "a, b, c")
 })
+
+test_that("str_flatten handles empty input with last", {
+  expect_equal(str_flatten(character(), ", ", " and "), "")
+  expect_equal(str_flatten(character(), "-"), "")
+})
+
+test_that("str_flatten handles single element with last", {
+  expect_equal(str_flatten("a", ", ", " and "), "a")
+  expect_equal(str_flatten("a", "-"), "a")
+})
+
+test_that("str_flatten handles all-NA input with na.rm", {
+  expect_equal(str_flatten(c(NA, NA), na.rm = TRUE), "")
+  expect_equal(str_flatten(NA_character_, na.rm = TRUE), "")
+})
+
+test_that("str_flatten_comma handles empty input with last", {
+  expect_equal(str_flatten_comma(character(), ", or "), "")
+  expect_equal(str_flatten_comma(character()), "")
+})
+
+test_that("str_flatten_comma handles single element with last", {
+  expect_equal(str_flatten_comma("a", ", or "), "a")
+  expect_equal(str_flatten_comma("a"), "a")
+})
+
+test_that("str_flatten_comma handles NA with last and na.rm", {
+  expect_equal(
+    str_flatten_comma(c("a", NA, "b"), ", or ", na.rm = TRUE),
+    "a, or b"
+  )
+  expect_equal(
+    str_flatten_comma(c(NA, "a", "b"), ", or ", na.rm = TRUE),
+    "a, or b"
+  )
+  expect_equal(
+    str_flatten_comma(c("a", "b", NA), ", or ", na.rm = TRUE),
+    "a, or b"
+  )
+})