json reader: correctly handle empty objects in objects

json reader: correctly handle empty objects in objects

Attempting to parse { "foo" : {} } previously resulted in an error.
(close-hash) expected to consolidate an object into the values of the
containing object (in the first line of code).  While there is none in
the case of the empty hash, the structure of the accumulator looked like
it contained an unconsolidated object, so it folded the values into the
keys and got very confused.