Support constructing without location

[vweevers]

I wanted to send a PR to level-hookdown, to replace memdb with level-mem, but tests failed because level-hookdown uses the following pattern:

var mem = require('memdb')
var db = mem({ valueEncoding: 'json' })

Which is a reasonable expectation. On level-mem you'd have to do:

var mem = require('level-mem')
var db = mem(null, { valueEncoding: 'json' })

This PR fixes that but it ain't pretty. Long-term we should perhaps move location into options, at least in leveldown and friends.