fs: invoke callbacks with undefined context

Many callbacks appear to be invoked with `this` set to `undefined`
including `fs.stat()`, `fs.lstat()`, and `fs.fstat()`.

However, some such as `fs.open()` and `fs.mkdtemp()` invoke their
callbacks with `this` set to `null`. Change to `undefined`.

lib/fs.js

@@ -132,7 +132,7 @@ function makeCallback(cb) {
   }
 
   return function() {
-    return cb.apply(null, arguments);
+    return cb.apply(undefined, arguments);
   };
 }
 

test/parallel/test-fs-mkdtemp.js

@@ -20,7 +20,7 @@ assert(common.fileExists(utf8));
 function handler(err, folder) {
   assert.ifError(err);
   assert(common.fileExists(folder));
-  assert.strictEqual(this, null);
+  assert.strictEqual(this, undefined);
 }
 
 fs.mkdtemp(path.join(common.tmpDir, 'bar.'), common.mustCall(handler));

test/parallel/test-fs-stat.js

@@ -66,7 +66,7 @@ fs.open('.', 'r', undefined, common.mustCall(function(err, fd) {
   // Confirm that we are not running in the context of the internal binding
   // layer.
   // Ref: https://github.com/nodejs/node/commit/463d6bac8b349acc462d345a6e298a76f7d06fb1
-  assert.strictEqual(this, null);
+  assert.strictEqual(this, undefined);
 }));
 
 // fstatSync