fix(models): use identity checks for output_type in MLXLM (#1997)

[Gopesh111]

Purpose

Fixes #1997 where MLXLMTypeAdapter.format_output_type and MLXLM.generate_batch used truthiness checks (if output_type: / if not output_type:) instead of explicit identity checks (if output_type is None: / if output_type is not None:).

Using boolean truthiness checks causes falsy custom logits processors or objects (such as structures defining __len__ = 0) to be wrongly evaluated as None or skip unsupported batch argument assertions.

Test Plan

Added unit test test_mlxlm_type_adapter_format_output_type_none_and_falsy in tests/models/test_mlxlm_type_adapter.py verifying that format_output_type(None) returns None and format_output_type(falsy_obj) returns [falsy_obj].

Run pytest locally:

pytest tests/models/test_mlxlm_type_adapter.py