docs(external docs): fix empty collection rendering by isset

[titaneric]

Summary

When I previewed the docs for #23935 , I noticed that there is some rendering error for null and []. Also, I found that there is a similar issue reported in #23860 . I try to fix them by isset Hugo function.

Vector configuration

How did you test this PR?

cd website
make quick-build
make serve

Root metadata path's returning result

remove VRL function example

split_path example in #23935

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

Closes: #23860

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[pront]

Thanks!