fix: Handle array of strings columns in Athena materialization

[alan-gauthier-jt]

What this PR does / why we need it

Fixes two related bugs that cause TypeError and ValueError when materializing
feature views with Array(String) columns using the Athena offline store.

Arrow/Athena deserializes Array(String) columns as numpy.ndarray (object dtype)
instead of plain Python lists. This breaks two code paths in type_map.py:

1. _convert_scalar_values_to_proto: pd.isnull(ndarray) returns an array of bools,
   and not <array> raises ValueError: The truth value of an empty array is ambiguous.
   → Already guarded by _is_array_like in newer Feast versions; no change needed here.

2. _convert_list_values_to_proto (generic list path): proto_type(val=ndarray) passes
   the raw numpy array to the protobuf constructor, which only accepts Python lists →
   TypeError: bad argument type for built-in operation. Additionally, Arrow nullable
   columns can yield None elements inside the ndarray, which StringList also rejects.

3. _validate_collection_item_types: None elements inside an ndarray failed the
   type(item) in valid_types check before reaching the sanitization step.

Changes

feast/type_map.py

• Add _to_proto_safe_list(value) helper that:

  • Calls .tolist() on any numpy.ndarray to produce a plain Python list
  • Replaces None elements with "" (empty string), which protobuf StringList accepts
  • Is a no-op for plain Python lists and scalar values

• Use _to_proto_safe_list in the generic list conversion (end of
  _convert_list_values_to_proto) instead of passing value directly to proto_type.

• Skip None elements in _validate_collection_item_types — None entries are valid
  in nullable Arrow columns and are sanitized by _to_proto_safe_list downstream; raising
  a TypeError on them before that point was incorrect.

Testing

Added TestArrowArrayStringListMaterialization in
sdk/python/tests/unit/test_type_map.py covering:

Test	Scenario
test_to_proto_safe_list_ndarray	ndarray → plain list
test_to_proto_safe_list_empty_ndarray	empty ndarray → empty list (was raising ValueError)
test_to_proto_safe_list_ndarray_with_none	None elements replaced with ""
test_to_proto_safe_list_plain_list	plain list passthrough
test_to_proto_safe_list_plain_list_with_none	None in plain list also replaced
test_to_proto_safe_list_scalar_passthrough	non-list values unchanged
test_string_list_from_ndarray	full round-trip via python_values_to_proto_values
test_string_list_from_empty_ndarray	empty ndarray no longer raises ValueError
test_string_list_from_ndarray_with_none_elements	None in ndarray no longer raises TypeError
test_string_list_null_row_produces_empty_proto	None rows produce empty ProtoValue
test_mixed_batch_simulating_athena_chunk	full simulation of a failing Athena materialization batch

pytest sdk/python/tests/unit/test_type_map.py::TestArrowArrayStringListMaterialization -v

Which issues this PR fixes

Fixes #6325

Does this PR introduce a user-facing change?

Yes — materialization of Array(String) (and other array-typed) feature columns from
Athena no longer fails with TypeError or ValueError when a batch contains empty
arrays, None rows, or None elements inside arrays.

Previously:
  TypeError: bad argument type for built-in operation
  ValueError: The truth value of an empty array is ambiguous

After this fix:
  Materialization completes successfully. None elements are stored as "".

---

[ntkathole]

@alan-gauthier-jt The approach used in https://github.com/feast-dev/feast/pull/6327/changes seems safer and preserve list length

[ntkathole]

if feast_value_type in _LIST_TYPE_NONE_REPLACEMENT instead?