Parameter initialization error when not setting rear_wheels_names and front_wheels_names

[dylan-gonzalez]

branch: feature/ackermann_drive_control

to use the four_wheel_steering_controller, the steers_names and wheels_names parameters need to be set. rear_wheels_names and front_wheels_names should not be set. (these parameters are a part of the steering_controllers_library)

But the steering_controllers_library parameter_listener initialization fails when not setting rear_wheels_names and front_wheels_names in the yaml config file.

To Reproduce
Steps to reproduce the behavior:

1. In your controllers.yaml file, set only wheels_names and steers_names - do not set rear_wheels_names and front_wheels_names.
2. Try to load the controller
3. See error: [gzserver-2] [INFO] [1700699681.934820805] [controller_manager]: Loading controller 'four_wheel_steering_controller' [gzserver-2] Exception thrown during controller's init with message: parameter '' has invalid type: expected [string_array] got [not set]

But when you set rear_wheels_names and front_wheels_names (and not wheels_names and steers_names), the controller works. Strange why leaving wheels_names and steers_names out doesn't throw an error.

I've tried removing the 'not_empty' validation rule from the parameter, but still the same error.