Add a row to document library like a custom list?

I would suggest you look on to the requirements first on what you are really trying to achieve, currently document libraries will need to have a document as an item before you can update its metadata.

How about maybe instead of document library, use a list, then add the files as attachments. You can use Pnp powerful library for list manipulations.

Or If you really would want it to be document library, you will need a 2 step process, on creating it in a list, then on approval or status / metadata change copy the list item to the document with the list metadata, more on combination of custom script and msflow.

Above is only a suggestions, so, Hope the concept helps on any way. Happy Coding! :)

A library is a location on a site where you can upload, create, update, and collaborate on files with team members.

It is not supported to add a row without using document/file uploading to document library.

You need to create list to add a row without using document/file uploading.

Reference:

https://support.microsoft.com/en-us/office/introduction-to-libraries-7d4221d9-8fb9-40d5-8441-2374c84b5e26

I recommend using PnP/PnPjs. First, you need to upload the file and after that, you can update the file item. Or you can simply make a list and write text into it.