net/textproto: fails to parse headers without a body

[kayrus]

What version of Go are you using (go version)?

$ go version
go version go1.18.3 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

See example: https://go.dev/play/p/q5lkQ1V0vBa

What did you expect to see?

I expect that mail.ReadMessage will return parsed headers with an empty body.

What did you see instead?

No body, no headers, just error:

2009/11/10 23:00:00 EOF

Program exited.

I'm not sure if this behavior aligns with RFC, but I receive emails with zero body from postfix, which should align with RFC.

[seankhliao]

Duplicate of #33823