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.

PR-URL: https://github.com/nodejs/node/pull/17644


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>