You must not change or falsify a copyright notice. So you can't just add your name here. However, if you modify the software in accordance to the license so that you have part of the copyright of the software, you may add a separate copyright notice like "Copyright 2018 Tom" above it.
Ideally, a copyright notice always names the copyright holder. This is important ~70 years from now to determine when the copyright expires. Without the name of the copyright holder this might be an anonymous work, which is still protected by copyright. This does not affect the validity of the license, and you can use it as normal. As the license explains, you must keep a copy of the copyright notice and the license in all versions of this software, even if you modify the code, even if you license your modifications differently.
You may want to contact the holder of the repository where you found that code to ask them to clarify this copyright notice. Probably, they themselves are the copyright holder. If they don't want to give their real name, it is fine if they use a pseudonym such as their GitHub user name. You would usually use open a GitHub issue for this, but you may also find their email address in the commits (you must clone the repository for this, as GitHub does not display the email address in their web interface).
