fix(ros2-bridge): don't panic on zero-field message types (std_msgs/Empty)

[phil-opp]

Summary

Fixes #2804.

Deserializing a ROS 2 message type with zero fields (e.g. std_msgs/msg/Empty) panicked in the ros2-bridge CDR→Arrow conversion:

called `Result::unwrap()` on an `Err` value: InvalidArgumentError("use StructArray::try_new_with_length or StructArray::new_empty_fields to create a struct array with no fields so that the length can be set correctly")

StructArray::from(Vec<..>) panics on an empty field vec because the array length is ambiguous with no columns. Since the deserializer runs inside the subscription take path while a RustDDS cache lock is held, the panic poisoned the DDS cache mutex and then killed the participant event loop — silently taking down every subscription on the participant (0 Hz on all topics) after a single stray Empty message.

Changes

• deserialize/mod.rs (the take path) and serialize/defaults.rs (default-value construction) now build a length‑1 empty‑fields struct via StructArray::new_empty_fields(1, None) when there are no fields, instead of fields.into(). This mirrors the pattern already used in the crate's default-value test code.
• Added a regression test empty_message_deserializes that deserializes a zero‑field message and asserts it produces a single struct row with no columns.

Testing

• cargo test -p dora-ros2-bridge-arrow — 16 passed, incl. the new regression test
• cargo clippy -p dora-ros2-bridge-arrow -- -D warnings — clean
• cargo fmt --all -- --check — clean

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[phil-opp]

Automated review by Claude — this is a fully automated review; no human has vetted it.

No issues found. StructArray::new_empty_fields(1, None) is the right way to build a zero-column struct with a defined length, and both the take-path (deserialize/mod.rs) and default-construction path (serialize/defaults.rs) are covered. The regression test exercises the real deserializer end-to-end on an empty CDR body and asserts a single zero-column row, so it genuinely guards the previously-panicking path.

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