GH-47447: [C++] Fix replace_with_mask for null type arrays

cpp/src/arrow/compute/kernels/vector_replace.cc

@@ -217,15 +217,15 @@ struct ReplaceMaskImpl<Type, enable_if_null<Type>> {
   static Result<int64_t> ExecScalarMask(KernelContext* ctx, const ArraySpan& array,
                                         const BooleanScalar& mask, ExecValue replacements,
                                         int64_t replacements_offset, ExecResult* out) {
-    out->value = array;
-    return Status::OK();
+    out->value = array.ToArrayData();
+    return replacements_offset;
   }
   static Result<int64_t> ExecArrayMask(KernelContext* ctx, const ArraySpan& array,
                                        const ArraySpan& mask, int64_t mask_offset,
                                        ExecValue replacements,
                                        int64_t replacements_offset, ExecResult* out) {
-    out->value = array;
-    return Status::OK();
+    out->value = array.ToArrayData();
+    return replacements_offset;
   }
 };
 

python/pyarrow/tests/test_compute.py

@@ -1987,6 +1987,26 @@ def test_fill_null_array(arrow_type):
     assert result.equals(expected)
 
 
+def test_replace_with_mask_null_type():
+    # GH-47447: replace_with_mask crashed for null type arrays
+    a = pa.array([None], pa.null())
+    b = pa.array([None], pa.null())
+
+    result = pc.replace_with_mask(a, True, b)
+    assert result.type == pa.null()
+
+    result = pc.replace_with_mask(a, False, b)
+    assert result.type == pa.null()
+
+    mask = pa.array([True])
+    result = pc.replace_with_mask(a, mask, b)
+    assert result.type == pa.null()
+
+    mask = pa.array([False])
+    result = pc.replace_with_mask(a, mask, b)
+    assert result.type == pa.null()
+
+
 @pytest.mark.parametrize('arrow_type', numerical_arrow_types)
 def test_fill_null_chunked_array(arrow_type):
     fill_value = pa.scalar(5, type=arrow_type)