Fix stream stalling on small files.

[BlakeB415]

Node: v17.0.0
OS: Windows 11

The end callback is never called on some smaller files causing the pipeline to stall. This seemed to fix it.

[timothyallan]

WHOOO What a lifesaver this was, thanks @BlakeB415. Spent ages trying to figure out why our dev boxes were hanging randomly with multer, but not production. This fixed it.

[AhmedAlSaey]

Can this PR be approved please?

[KostyaTretyak]

Although I don't have deep knowledge in streams, but I feel that the following example will be more correct:

_flush(cb) {
    this[kResult].finally(() => {
      if (this[kStream] != null) {
        this[kStream].end();
      }
      cb(null);
    });
  }