High CPU usage with int32_publisher

[yashi]

Describe the bug
Having only timers on an executor makes rmw_wait() to spin without blocking. This causes high cpu usage and it's a problem on a RTOS with threading support.

To Reproduce
Just build int32_publisher from micro-ROS-demos.

Expected behaviour
rclc_executor_spin() should always be a blocking call without any service, client, subscription, or timer.

System information (please complete the following information):

• OS: Zephyr (but it shouldn't matter)
• ROS 2: Humble (but it shouldn't matter)
• Version: humble.

Additional context
https://micro-ros.slack.com/archives/CNC141G0N/p1684752941054099

[Acuadros95]

This is resolved by #305, please close when merged

[yashi]

Thanks for you PR.

BTW, if you have "Resolves #297" in the description of #305, this will be closed when merged.
https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword