A flow mapping entry may begin with its `:` indicator for an empty key
(`{ : value }`) or carry only properties before the `:` (`{ !!str : value }`).
Both resolve to a null key, but parseFlowMappingPair required a node before
the `:` and raised "expected a YAML node".
Emit a null key in that position instead, mirroring the empty-value handling
already present after the `:`. The change consumes the key's properties first
so a property-only key keeps its tag or anchor, and is confined to flow
mapping key position, where a null key is unambiguously valid; an entry with
no `:` at all (`{ a, }`) still errors.
Raises yaml-test-suite parse-level to 297/333 (89.2%) and value-level to
174/192, with the should-error rejection rate unchanged (no invalid document
newly accepted). 422 tests pass.
