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-typed columns (e.g. Array(String), Array(Int64)) using
the Athena offline store.

Arrow/Athena deserializes array 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 protobuf repeated fields
   also reject.

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 module-level _LIST_NONE_DEFAULTS dict mapping each list ValueType to a
  type-appropriate zero/empty default value used to replace None elements:

  • STRING_LIST, UUID_LIST, TIME_UUID_LIST, DECIMAL_LIST → ""
  • BYTES_LIST → b""
  • INT32_LIST, INT64_LIST → 0
  • FLOAT_LIST, DOUBLE_LIST → 0.0
  • BOOL_LIST → False
  • UNIX_TIMESTAMP_LIST → NULL_TIMESTAMP_INT_VALUE

• Add module-level _sanitize_list_value(value, feast_value_type) helper that:

  • Calls .tolist() on any numpy.ndarray to produce a plain Python list
    (empty ndarray → None, treated as a missing row)
  • Replaces None elements with the type-appropriate default from _LIST_NONE_DEFAULTS
  • Is a no-op for plain Python lists without None and for scalar values

• Apply sanitization upfront in _convert_list_values_to_proto: both values and
  sample are normalised via _sanitize_list_value before any type-checking or proto
  conversion, removing the need for per-path ndarray handling.

• Remove the old _to_proto_safe_list / _DROP_NONE / _LIST_TYPE_NONE_REPLACEMENT
  module-level helpers, which have been superseded by the above.

• Skip None elements in _validate_collection_item_types — None entries are
  valid in nullable Arrow columns and are sanitized upstream; 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_sanitize_list_value_ndarray	ndarray → plain list
test_sanitize_list_value_empty_ndarray	empty ndarray → None (missing row)
test_sanitize_list_value_ndarray_with_none	None elements in STRING_LIST replaced with ""
test_sanitize_list_value_plain_list	plain list passthrough
test_sanitize_list_value_plain_list_with_none	None in plain STRING_LIST list replaced with ""
test_sanitize_list_value_numeric_none_replaced	None in numeric/bool lists replaced with zero default
test_sanitize_list_value_bytes_none_replaced	None in BYTES_LIST replaced with b""
test_sanitize_list_value_scalar_passthrough	non-list, non-ndarray 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-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. None elements inside an array are now stored as the
type-appropriate zero/empty value (e.g. "" for strings, 0 for integers).

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 inside arrays are replaced with type-appropriate defaults.

[ntkathole]

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

[alan-gauthier-jt]

I changed the implementation to a similar solution than #6327

[ntkathole]

if feast_value_type in _LIST_TYPE_NONE_REPLACEMENT instead?