`toString()` incorrectly converts comment to string

[RedCMD]

Describe the bug
similar to #686
toString() incorrectly converts comment to string when at the start of a block-scalar with a indent indicator
and multiple newline lines are added?

To Reproduce

|5
#comment

parsedDocumentYaml.toString()

Expected behaviour
comment to stay a comment

Versions:

• Environment: VSCode 1.121.0 Windows
• yaml: 2.7.1

Additional context

[dyk1454683243-sudo]

Taking this: empty document-root block scalar (| / >) followed by a document comment should round-trip — toString() needs to keep an indent indicator so the comment stays a comment, not scalar content.

There is no open Fixes PR (prior #689 / #697 are closed). I'll keep the change root-only and minimal.

[dyk1454683243-sudo]

Opened #721 for this.