Fix gather() for STRUCT inputs with no nulls in members. (#9194)

Fixes #9188.

Prior to this fix, when `cudf::gather()` is called on a STRUCT input
column, the null masks of the children of the result column would not
be set correctly if the child columns do not contain nulls.

This fix enforces null mask calculation if `NULLIFY` is set.

In addition, this commit also cleans up the `TypedStructGatherTest` test suite:
1. IIFEs for `STRUCT` column construction.
2. Corrections for `assert()` conditions.
3. Switched `column` construction to `column_wrapper` and `column_view`.
4. Corrected the output null mask for `TestGatherStructOfListOfStructs`.
5. Added repro test for #9188.

Authors:
  - MithunR (https://github.com/mythrocks)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Jason Lowe (https://github.com/jlowe)
  - Karthikeyan (https://github.com/karthikeyann)

URL: #9194

cpp/include/cudf/detail/gather.cuh

@@ -470,9 +470,10 @@ struct column_gatherer_impl<struct_view> {
                                                                          mr);
                    });
 
-    auto const nullable = std::any_of(structs_column.child_begin(),
-                                      structs_column.child_end(),
-                                      [](auto const& col) { return col.nullable(); });
+    auto const nullable =
+      nullify_out_of_bounds or std::any_of(structs_column.child_begin(),
+                                           structs_column.child_end(),
+                                           [](auto const& col) { return col.nullable(); });
     if (nullable) {
       gather_bitmask(
         // Table view of struct column.