The block mapping parser required a ':' after every explicit key ('? key'),
so a key with no value, including several '? key' entries in a row (a set),
failed to parse with 'expected mapping value'. An explicit key with no
following ':' now takes an implicit null value, shared with the existing
'key:'-with-nothing-after path via one emitNullMappingValue helper.
Explicit keys with values and complex keys in general are unaffected. A
test that asserted '? [a, b]' followed by a block entry is a missing-value
error was corrected: the key is a valid null entry, and the trailing
sequence is now rejected as extra content (a second root node), which is
the more accurate diagnosis.
Measured against the official YAML test-suite, parse-level conformance
rises from 229/333 (68.8%) to 235/333 (70.6%), with several of the newly
parsed documents also value-correct and no regression.
