GH-48832: [R] Fix crash with zero-length POSIXct tzone attribute (#49619)

### Rationale for this change

In R 4.5.2+, `as.POSIXct(x = NULL)` creates a zero-length `POSIXct` with `INTSXP` (integer) type, not `REALSXP` (double). The `GetVectorType()` function in `r_to_arrow.cpp` only checked for `POSIXct` in the `REALSXP` branch, so it misclassified zero-length `POSIXct` as `INT32`.

### What changes are included in this PR?

Map any `POSIXct` object to correct class by adding the check to the `INTSXP` branch of `GetVectorType()`

### Are these changes tested?

Yes

### Are there any user-facing changes?

No
* GitHub Issue: #48832

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

Author: thisisnic

r/src/arrowExports.cpp

@@ -93,6 +93,8 @@ RVectorType GetVectorType(SEXP x) {
         return FACTOR;
       } else if (Rf_inherits(x, "Date")) {
         return DATE_INT;
+      } else if (Rf_inherits(x, "POSIXct")) {
+        return POSIXCT;
       }
       return INT32;
     case STRSXP:

r/src/r_to_arrow.cpp

@@ -324,6 +324,17 @@ test_that("array uses local timezone for POSIXct without timezone", {
   })
 })
 
+test_that("zero-length POSIXct can be converted (GH-48832)", {
+  # In R 4.5.2+, zero-length POSIXct vectors are integer type, not double
+  x <- as.POSIXct(x = NULL)
+
+  # Should behave the same as non-empty POSIXct with empty tzone
+  expect_type_equal(infer_type(x), timestamp("us"))
+  arr <- Array$create(x)
+  expect_equal(arr$length(), 0L)
+  expect_type_equal(arr, timestamp("us"))
+})
+
 test_that("Timezone handling in Arrow roundtrip (ARROW-3543)", {
   # Write a feather file as that's what the initial bug report used
   df <- tibble::tibble(

r/tests/testthat/test-Array.R

@@ -129,6 +129,16 @@ test_that("write_parquet() can truncate timestamps", {
   expect_equal(as.data.frame(tab), as.data.frame(new))
 })
 
+test_that("write_parquet() works with zero-length POSIXct (GH-48832)", {
+  # In R 4.5.2+, zero-length POSIXct vectors are integer type, not double
+  tf <- tempfile()
+  on.exit(unlink(tf))
+
+  expect_no_error(write_parquet(data.frame(x = as.POSIXct(x = NULL)), tf))
+  result <- read_parquet(tf)
+  expect_equal(nrow(result), 0)
+})
+
 test_that("make_valid_parquet_version()", {
   expect_equal(
     make_valid_parquet_version("1.0"),