Is it safe to delete all revisions of a deleted node? I'm trying to import nodes from a Drupal 7 database to a brand new Drupal 8 one using the Feeds module.

I'v imported stuff and then deleted it (because I'm testing at the moment the best way to do it). I keep deleting nodes, but I've noticed that the revision tables are full of stuff.

Currently there is no node in the site, but the revision tables are occupying 50MB already.

Is it safe to empty all those tables?
I hope you are doing your tests on a test server. In that case, yes, they are safe to be emptied, even if it would be better to let Drupal and any third-party module to remove any entity data they have in their tables.

Rather than worrying to empty tables, I would find a quicker way to create a test server that is then destroyed when the test is done.