Change nullable() to has_nulls() in cudf::detail::gather

cpp/include/cudf/detail/gather.cuh

@@ -15,6 +15,7 @@
  */
 #pragma once
 
+#include <cstddef>
 #include <cudf/detail/copy.hpp>
 #include <cudf/detail/indexalator.cuh>
 #include <cudf/detail/null_mask.hpp>
@@ -678,9 +679,24 @@ std::unique_ptr<table> gather(table_view const& source_table,
                           return col.nullable();
                         });
   if (nullable) {
-    auto const op = bounds_policy == out_of_bounds_policy::NULLIFY ? gather_bitmask_op::NULLIFY
-                                                                   : gather_bitmask_op::DONT_CHECK;
-    gather_bitmask(source_table, gather_map_begin, destination_columns, op, stream, mr);
+    auto const has_nulls =
+      bounds_policy == out_of_bounds_policy::NULLIFY ||
+      std::any_of(
+        source_table.begin(), source_table.end(), [](auto const& col) { return col.has_nulls(); });
+    if (has_nulls) {
+      auto const op = bounds_policy == out_of_bounds_policy::NULLIFY
+                        ? gather_bitmask_op::NULLIFY
+                        : gather_bitmask_op::DONT_CHECK;
+      gather_bitmask(source_table, gather_map_begin, destination_columns, op, stream, mr);
+    } else {
+      for (size_type i = 0; i < source_table.num_columns(); ++i) {
+        if (source_table.column(i).nullable() and not source_table.column(i).has_nulls()) {
+          auto mask = detail::create_null_mask(
+            destination_columns[i]->size(), mask_state::ALL_VALID, stream, mr);
+          destination_columns[i]->set_null_mask(std::move(mask), 0);
+        }
+      }
+    }
   }
 
   return std::make_unique<table>(std::move(destination_columns));