Is there any harm of leaving Ethernet outlet unplugged?

First: you are safe.

I'm assuming you have the standard plug that looks like this:

but if it is a wire that plugs into a laptop or desktop computer, the chances are good that it is a standard RJ45 outlet.

In that case, while the device on the other end could supply power to the port,

1. it probably does not for most home devices / installations, and
2. the power supplied even in that case is quite limited, and
3. there is a sophisticated process to detect whether to supply power to the port, and people don't qualify. Also, in this case, there is no laser / radiation at all.

If you are using fibre, you are still safe. A fibre port does output laser light (see here), but it is a "Class 1" laser (see here), so you'll have to try kind of hard to get hurt by it. Plus, if it is fibre, there is absolutely no electrical hazard (fibre doesn't conduct electricity)

It is always reasonable to be interested in safety, and to seek out information to ensure your safety. Keep up the good work.

The question I'd be asking is whether your ethernet is safe from your kitchen? You wouldn't want to splash liquid into the port and cause corrosion of the socket, as then devices plugged in there in the future might not work. To protect the socket, search for a cheap RJ45 Dust Cover on Amazon/Ebay/your local computer store.

Another 'fix' is to take a old ethernet cable with a RJ45 plug on it, and cut it off leaving a inch or so of cable. Plug this into the port to keep dirt/dust out.