fix: Prevent empty values in toc.yaml fields

[astandrik]

Description

• Enforce non-empty values in toc.yaml by extending TOC validation to treat empty strings in scalar fields (name, href, title, label, navigation) as errors.
• Preserve existing error format for TOC structure issues while adding a specific message for empty values (e.g. empty value is not allowed) including the precise field path (e.g. items[2].items[0].name).
• Add regression coverage via a new e2e fixture and test that verify builds fail with code 1 and report the expected error messages when toc.yaml contains empty values.

Related issue: fixes #1399

[astandrik]

@martyanovandrey would appreciate your review here