stream: readable stream should not push undefined in non-objectMode

[MoonBall]

From the doc:

But current Node.js allows we to push undefined when stream is in non-object mode.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

stream

[mcollina]

Why do we need it? I'm not understanding the reasoning behind this.

[MoonBall]

Current Node.js allows we to push undefined when stream is in non-object mode.
I think pushing undefined should not be allowed when stream is in non-object mode, so I made this change.

[addaleax]

@MoonBall It would be helpful if you could explain why you think that that should not be allowed?

[MoonBall]

There is no obvious reason. just feeling.
Why do we allow it?
Is the reason that we treat undefined to empty string or buffer?
If so, I can also accept it.

[MoonBall]

@addaleax If we allow it, I can add a test case and a comment to explain the reason.
It surely gives me some confusion.

[MoonBall]

@addaleax I found the doc.

[MoonBall]

@mcollina @addaleax I found that readable.push() supported undefined in normal mode long ago. The PR can be closed, and I will modify the doc to state it.

[mcollina]

Thanks for the investigation!