remove defined behavior for nullish targets

By targets I mean range options as well as seek targets.

Previously, nullish range options were ignored and `seek(null)`
and `seek(undefined)` would throw an error. Now they translate to
`String(null)` and `String(undefined)`. This means `{ gt: null }`
and `{ gt: undefined }` are *not* the same as `{}`.

This change makes it explicit that leveldown only supports buffers
and strings. Nullish targets do have a meaning in the ecosystem;
that meaning should be given at a higher level like encoding-down.

In other words, it isn't leveldown's concern anymore.

leveldown.js

@@ -29,13 +29,12 @@ LevelDOWN.prototype._close = function (callback) {
   this.binding.close(callback)
 }
 
-LevelDOWN.prototype._serializeKey =
-LevelDOWN.prototype._serializeValue = function (datum) {
-  // This is only needed for range options. We must ignore nullish
-  // range options and already ignore empty range options in C++.
-  if (datum == null) return ''
+LevelDOWN.prototype._serializeKey = function (key) {
+  return Buffer.isBuffer(key) ? key : String(key)
+}
 
-  return Buffer.isBuffer(datum) ? datum : String(datum)
+LevelDOWN.prototype._serializeValue = function (value) {
+  return Buffer.isBuffer(value) ? value : String(value)
 }
 
 LevelDOWN.prototype._put = function (key, value, options, callback) {

test/iterator-test.js

@@ -4,7 +4,7 @@ const lexi = require('lexicographic-integer')
 const util = require('util')
 
 make('iterator#seek throws if target is empty', function (db, t, done) {
-  var targets = [null, undefined, '', Buffer.alloc(0), []]
+  var targets = ['', Buffer.alloc(0), []]
   var pending = targets.length
 
   targets.forEach(function (target) {