stream: remove undefined check

`validChunk` allowed `undefined` as a chunk in object mode; however,
this was redundant, since:

- `validChunk()` is only used by `.write()`
- If the `validChunk()` check passes for `undefined`, `.write()`
  calls `writeOrBuffer()`
- If `writeOrBuffer()` does not receive a Buffer, it calls
  `decodeChunk()`
  - `decodeChunk()` ignores `undefined` because it checks
    `typeof chunk === 'string'`
  - After that call, `chunk.length` is accessed, which throws an
    error if `chunk` is undefined`.

This “fixes” a bug in the sense that `state.pendingcb` is no longer
incremented for write attempts that fail like this.

lib/_stream_writable.js

@@ -249,9 +249,7 @@ function validChunk(stream, state, chunk, cb) {
 
   if (chunk === null) {
     er = new errors.TypeError('ERR_STREAM_NULL_VALUES');
-  } else if (typeof chunk !== 'string' &&
-             chunk !== undefined &&
-             !state.objectMode) {
+  } else if (typeof chunk !== 'string' && !state.objectMode) {
     er = new errors.TypeError('ERR_INVALID_ARG_TYPE', 'chunk',
                               ['string', 'Buffer']);
   }