fix: avoid panic when a parameter override is an empty YAML array

[Lots-ninety-nine]

Fixes #636.

Problem

A parameter override declared as an empty YAML array crashes the node at executor.create_node instead of returning a recoverable error.

params.yaml:

minimal_publisher:
  ros__parameters:
    my_param: []

$ ros2 run examples_rclrs_minimal_pub_sub minimal_publisher \
    --ros-args --params-file params.yaml

thread 'main' panicked at rclrs/src/parameter/value.rs:458:9:
assertion `left == right` failed
  left: 0
 right: 1
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed
   4: rclrs::parameter::value::ParameterValue::from_rcl_variant
   5: rclrs::parameter::override_map::resolve_parameter_overrides
   6: rclrs::parameter::ParameterInterface::new
   7: rclrs::node::node_options::NodeOptions::build
   8: rclrs::executor::Executor::create_node
   9: minimal_publisher::main

Root cause

ParameterValue::from_rcl_variant expects exactly one of the nine type-specific pointers on rcl_variant_t to be non-null. For an empty YAML array the rcl YAML parser cannot infer the element type, so it emits a variant with all nine of those pointers null. The function then hits assert_eq!(num_active, 1) and panics.

The same from_rcl_variant is reached for every parameter override that the node sees, so any node that is started with a parameters file containing a single [] entry will fail to start at all — including nodes that do not even declare that parameter.

Fix

Change from_rcl_variant to return Option<Self> and return None when the variant has zero (or, hypothetically, more than one) active fields. The single in-tree caller — resolve_parameter_overrides in override_map.rs — now skips overrides whose type could not be determined:

if let Some(value) = ParameterValue::from_rcl_variant(variant) {
    map.insert(param_name, value);
}

Rationale for silently dropping rather than surfacing a brand new error variant:

• The node still starts, which matches the behavior users expect when most of their parameters file is well-formed.
• If a node actually requires the ambiguous parameter, the existing DeclarationError::NoValueAvailable will fire at declare_parameter time and point at the specific parameter, which is more actionable than a panic during construction.
• The change keeps from_rcl_variant pub(crate) and does not add a new variant to RclrsError, so the public API surface is unchanged.

The existing in-module test for from_rcl_variant is adjusted to .expect(...) since the variants it constructs are always well-formed.

Verification

Unit tests

Added a regression test test_resolve_parameter_overrides_skips_empty_array that loads:

/my_ns/my_node:
    ros__parameters:
        good_int: 7
        empty_array: []

and asserts:

• good_int is present with value Integer(7),
• empty_array is not in the override map,
• no panic occurs.

Full rclrs test suite locally:

test result: ok. 119 passed; 0 failed; 0 ignored; 0 measured

End-to-end

With the same params.yaml from the bug report, after the fix:

$ ros2 run examples_rclrs_minimal_pub_sub minimal_publisher \
    --ros-args --params-file params.yaml
Publishing: [Hello, world! 1]
Publishing: [Hello, world! 2]
Publishing: [Hello, world! 3]
...

Tested on:

• Ubuntu 24.04 (Noble), ROS 2 Jazzy
• rustc 1.86 stable, cargo-ament-build 0.1