Handle null showtime screen formats

[tducret]

Summary

• ignore null entries in Allociné showtime experience and projection arrays
• add a VCR cassette and snapshot reproducing the P0535 response with projection: [null]
• document the requirement to use recorded upstream responses for parser tests

Testing

• uv run pytest
• uv run ruff check .
• uv run ruff format --check .
• uv run ty check