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.
