Fix file stream consumption when buffer has not loaded yet

[andersonjoseph]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

---

Hey! This PR fixes #369

Seems like _buf is initialized with null and its data is being loaded through part.toBuffer(). The condition that checks if the field has data in its buffer compares only against undefined instead of null, leading to an exception when calling field._buf.toString() when _buf === null.

Let me know if I'm missing something or if something needs to be changed.

[mcollina]

lgtm