Control-click on the Library's icon in the Movies folder (or wherever you have it stored) and choose Get Info from the pop up menu. If all of the media files have been deleted from the library the Get Info should show very few bytes.
Deleting all of the media out of the library will render all of the projects useless. The projects in the iMovie app will display black thumbnails with a ? mark in them and the notation "missing file". I doubt that you will be able to repopulate the projects. If you are going to delete all of the media in the library you might as well delete the entire library itself and create a new library for going forward. Just first export all of your "keep" projects out as movies so that you will have some archive of what you have done.
As I mentioned in an earlier post, the safest thing would be to move your library to an external drive that is formatted Max OS Extended (Journaled) or APFS. That would free up space on your internal drive while allowing you to keep all of your projects intact. That is what I recommend.
-- Rich