Fix three should-parse conformance gaps: a repeated document-end marker (M7A3),
a root block scalar ended by a document marker (W4TN), and an empty explicit key
in a flow mapping (DFF7). Threaded the located parse end mark through the new
explicit-key emit. Parse-level suite conformance reaches 318/333.
An explicit key (`?`) may have no key node: `? : v`, a trailing `? }` / `? ]` /
`? ,`, or `?` at the end of a flow mapping all denote a null key. The parser now
emits an empty key in those positions instead of failing to find a node before
the value indicator or terminator.
Fixes suite case DFF7 (parse-level conformance 316 -> 317); adds tests.
