URL('file:///') should be absolute url without netloc part

[asvetlov]

Now it is relative which is confusing

[aio-libs-bot]

GitMate.io thinks possibly related issues are #170 (Don't encode netloc if all netloc parts are empty), #172 (Replace _val with separate props for url parts), #138 (Question: how could I join to URL list path parts?), #272 (bool(URL()) should be False), and #95 (Passwords without users).