Improve `block.body` return on absence of content

Description

Per the code at https://github.com/typst/typst/blob/ddd8ae7f49d7adf35702fd9347d53fdd726ead55/crates/typst-library/src/layout/container.rs#L428-L435

block.body returns auto when the block was created internally, without any content. While this may seem harmless at first since those blocks are internal, this behavior is observable if you write e.g.

#show block: it => it.body

#line()

This prints auto, which shouldn't be possible as a block's body should always be content or none per the docs.

While you're normally not supposed to write such rules and anything goes, some workarounds for some particular issues use show block - this was found by @jassielof when trying to use https://github.com/typst/typst/issues/2639#issuecomment-2994374209, leading to errors as auto.func() doesn't exist.

Perhaps it should return some form of opaque content to mean "internal stuff, don't touch" instead of auto to remain consistent with the documented type for body?

Reproduction URL

No response

Operating system

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Typst version

• I am using the latest version of Typst

Inspect crates/typst-library/src/layout/container.rs at lines 428-435, then reproduce the issue with the provided show block rule and line() example. Determine how an internally created block with no content should satisfy the documented body type, and verify that the result no longer exposes auto.