hyperdb.del() should not add null value for already deleted or non-existent keys

Currently deleting keys adds a null value to the db regardless of if the key exists or not. It would be great to check if the key exists or not before deleting to avoid unnecessary data in the db.

I wrote some tests for this the first and last are currently failing.

tape('does not delete non existent keys', function (t) {
  var db = create.one()
  db.del('hello', function (err, node) {
    t.error(err, 'no error')
    t.same(db._writers[0].length(), 0)
    t.end()
  })
})

tape('does not delete non existent keys', function (t) {
  var db = create.one()
  db.put('hello', 'world', function (err, node) {
    t.error(err, 'no error')
    db.del('hello', function (err, node) {
      t.error(err, 'no error')
      t.same(db._writers[0].length(), 2)
      t.end()
    })
  })
})


tape('does not delete non existent keys', function (t) {
  var db = create.one()
  db.put('hello', 'world', function (err, node) {
    t.error(err, 'no error')
    db.del('hello', function (err, node) {
      t.error(err, 'no error')
      t.same(db._writers[0].length(), 2)
      db.del('hello', function (err, node) {
        t.error(err, 'no error')
        t.same(db._writers[0].length(), 2)
        t.end()
      })
    })
  })
})

Start at hyperdb.del() and run the three tape tests included in the issue. Check the writer length after deleting a missing key, deleting an existing key, and deleting the same key twice; done means no null value is added for missing or already deleted keys while the expected single deletion remains recorded.