Original PR: https://github.com/ytdl-org/youtube-dl/pull/30229
Per W3C JSON-LD v1.1 §4.9 (non-normative ref):
When a JSON-LD document's top-level structure is a map that contains
no other keys than @graph and optionally @context (properties that
are not mapped to an IRI or a keyword are ignored), @graph is
considered to express the otherwise implicit default graph.
Authored by: zmousm