pgsql: Remove ill-conceived ban on zero length json object keys.

Remove ill-conceived ban on zero length json object keys.

We removed a similar ban on this in json_object recently, but the ban in
datum_to_json was left, which generate4d sprutious errors in othee json
generators, notable json_build_object.

Along the way, add an assertion that datum_to_json is not passed a null
key. All current callers comply with this rule, but the assertion will
catch any possible future misbehaviour.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/d99d0e60eaa8334b698a070cb919202607660a65

Modified Files
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src/backend/utils/adt/json.c |    7 +++----
1 file changed, 3 insertions(+), 4 deletions(-)