fs: fix cb/sync writev empty array behavior

lib/fs.js

@@ -906,6 +906,11 @@ function writev(fd, buffers, position, callback) {
   validateBufferArray(buffers);
   callback = maybeCallback(callback || position);
 
+  if (buffers.length === 0) {
+    process.nextTick(callback, null, 0, buffers);
+    return;
+  }
+
   const req = new FSReqCallback();
   req.oncomplete = wrapper;
 
@@ -932,6 +937,10 @@ function writevSync(fd, buffers, position) {
   fd = getValidatedFd(fd);
   validateBufferArray(buffers);
 
+  if (buffers.length === 0) {
+    return 0;
+  }
+
   const ctx = {};
 
   if (typeof position !== 'number')

test/parallel/test-fs-writev-sync.js

@@ -56,11 +56,21 @@ const getFileName = (i) => path.join(tmpdir.path, `writev_sync_${i}.txt`);
   assert(Buffer.concat(bufferArr).equals(fs.readFileSync(filename)));
 }
 
+// fs.writevSync with empty array of buffers
+{
+  const filename = getFileName(3);
+  const fd = fs.openSync(filename, 'w');
+  const written = fs.writevSync(fd, []);
+  assert.strictEqual(written, 0);
+  fs.closeSync(fd);
+
+}
+
 /**
  * Testing with wrong input types
  */
 {
-  const filename = getFileName(3);
+  const filename = getFileName(4);
   const fd = fs.openSync(filename, 'w');
 
   [false, 'test', {}, [{}], ['sdf'], null, undefined].forEach((i) => {

test/parallel/test-fs-writev.js

@@ -57,11 +57,30 @@ const getFileName = (i) => path.join(tmpdir.path, `writev_${i}.txt`);
   fs.writev(fd, bufferArr, done);
 }
 
+
+// fs.writev with empty array of buffers
+{
+  const filename = getFileName(3);
+  const fd = fs.openSync(filename, 'w');
+  const bufferArr = [];
+  let afterSyncCall = false;
+
+  const done = common.mustSucceed((written, buffers) => {
+    assert.strictEqual(buffers.length, 0);
+    assert.strictEqual(written, 0);
+    assert(afterSyncCall);
+    fs.closeSync(fd);
+  });
+
+  fs.writev(fd, bufferArr, done);
+  afterSyncCall = true;
+}
+
 /**
  * Testing with wrong input types
  */
 {
-  const filename = getFileName(3);
+  const filename = getFileName(4);
   const fd = fs.openSync(filename, 'w');
 
   [false, 'test', {}, [{}], ['sdf'], null, undefined].forEach((i) => {