Fix strings::detail::copy_range when target contains nulls (#16626)

Fixes the logic in `cudf::strings::detail::copy_range` handling of nulls in the target range. The optimization check for nulls is removed simplifying the logic and making it more reliable as well. The benchmark showed no significant change in performance.
Also adds a specific gtest for this case.
Error was introduced in #15010
Closes #16618

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Bradley Dice (https://github.com/bdice)

URL: #16626

cpp/src/strings/copying/copy_range.cu

@@ -40,20 +40,14 @@ struct compute_element_size {
   size_type source_begin;
   size_type target_begin;
   size_type target_end;
-  bool source_has_nulls;
-  bool target_has_nulls;
 
   __device__ cudf::size_type operator()(cudf::size_type idx)
   {
     if (idx >= target_begin && idx < target_end) {
       auto const str_idx = source_begin + (idx - target_begin);
-      return source_has_nulls && d_source.is_null_nocheck(str_idx)
-               ? 0
-               : d_source.element<string_view>(str_idx).size_bytes();
+      return d_source.is_null(str_idx) ? 0 : d_source.element<string_view>(str_idx).size_bytes();
     } else {
-      return target_has_nulls && d_target.is_null_nocheck(idx)
-               ? 0
-               : d_target.element<string_view>(idx).size_bytes();
+      return d_target.is_null(idx) ? 0 : d_target.element<string_view>(idx).size_bytes();
     }
   }
 };
@@ -97,20 +91,9 @@ std::unique_ptr<column> copy_range(strings_column_view const& source,
       mr);
   }();
 
-  auto [check_source, check_target] = [target, null_count = null_count] {
-    // check validities for both source & target
-    if (target.has_nulls()) { return std::make_pair(true, true); }
-    // check validities for source only
-    if (null_count > 0) { return std::make_pair(true, false); }
-    // no need to check validities
-    return std::make_pair(false, false);
-  }();
-
   // create offsets
   auto sizes_begin = cudf::detail::make_counting_transform_iterator(
-    0,
-    compute_element_size{
-      d_source, d_target, source_begin, target_begin, target_end, check_source, check_target});
+    0, compute_element_size{d_source, d_target, source_begin, target_begin, target_end});
   auto [offsets_column, chars_bytes] = cudf::strings::detail::make_offsets_child_column(
     sizes_begin, sizes_begin + target.size(), stream, mr);
   auto d_offsets = cudf::detail::offsetalator_factory::make_input_iterator(offsets_column->view());

cpp/tests/copying/copy_range_tests.cpp

@@ -232,6 +232,16 @@ TEST_F(CopyRangeTestFixture, CopyWithNullsString)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(*p_ret, expected);
 }
 
+TEST_F(CopyRangeTestFixture, CopyWithTargetNullsString)
+{
+  auto target =
+    cudf::test::strings_column_wrapper({"a", "b", "", "d", "", "é"}, {1, 1, 0, 1, 1, 1});
+  auto source   = cudf::test::strings_column_wrapper({"A", "B", "C", "D", "E", "F"});
+  auto result   = cudf::copy_range(source, target, 1, 5, 1);
+  auto expected = cudf::test::strings_column_wrapper({"a", "B", "C", "D", "E", "é"});
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(result->view(), expected);
+}
+
 TEST_F(CopyRangeTestFixture, CopyNoNullsString)
 {
   cudf::size_type size{100};