Fix Set Producer metadata on saved document when provided

[guidimas]

What?

When provided, Producer info is stored in the saved document.

Why?

Producer info was getting overwritten before saving the document, even when setProducer was called before.

How?

Modified PDFDocument.ts to use the default value for Producer only when this information was not provided or updated using setProducer.

Testing?

PDFDocument unit test updated to cover this scenario.

Screenshots

N/A

Suggested Reading?

Yes

Anything Else?

N/A

Checklist

• I read CONTRIBUTING.md.
• I read MAINTAINERSHIP.md#pull-requests.
• I added/updated unit tests for my changes.
• I added/updated integration tests for my changes.
• I ran the integration tests.
• I tested my changes in Node, Deno, and the browser.
• I viewed documents produced with my changes in Adobe Acrobat, Foxit Reader, Firefox, and Chrome.
• I added/updated doc comments for any new/modified public APIs.
• My changes work for both new and existing PDF files.
• I ran the linter on my changes.