The multimodal processor in bart.py broke under vLLM 0.18 in three places:

- TextDataParser relied on MultiModalDataParser._is_empty, which was
  removed in 0.18. Replaced with inline emptiness checks for str and list.

- create_encoder_prompt previously tokenized `prompt` as the encoder text.
  In 0.18 `inputs.prompt` passed to this method is the DECODER prompt text,
  not the encoder text (the encoder content lives in mm_data). The method
  now returns a single [0] placeholder token; _get_prompt_updates replaces
  it with the correct number of encoder token slots during rendering.

- _call_hf_processor is now sometimes called with an already-tokenized
  decoder prompt (list[int]) instead of a str. Handle both cases when
  building result["input_ids"].

Adds tests/test_vllm_018_compat.py with three narrow unit tests covering
each of these paths; no GPU required.

Signed-off-by: David Schulmeister <dschulmeist@users.noreply.github.com>