how to convert json object to array

jq-object-to-array

# wanted result:
# [
#  [ "foo", "bar", "buz", "xx" ],
#  [ "a1",  "b1",  "c1",  null ],
#  [ "a2",  "b2",  "c2",  null ],
#  [ "a3",  "b3",  "c3",  null ],
#  [ "a4",  "b4",  "c4",  "undef" ]
# ]

printf '[
{
  "foo": "a1",
  "bar": "b1",
  "buz": "c1"
}, {
  "foo": "a2",
  "bar": "b2",
  "buz": "c2"
}, {
  "foo": "a3",
  "bar": "b3",
  "buz": "c3"
}, {
  "xx" : "undef",
  "foo": "a4",
  "bar": "b4",
  "buz": "c4"
}
]' |
jq '    {
                "data": .,
                "keys": (.|map(keys)|flatten(1)|unique)
        } |
                .data |= map(
                        to_entries|sort_by(.key)|map(.value)|flatten(1)
                ) |
        [ [.keys|sort], .data ]
        | flatten(1)
'

# 1) get the keys
#   map(keys)|flatten(1)|unique
#   map(keys)|flatten(1)|unique|sort

# 2) make a array of array with each value ...
#   ?
#   map( to_entries|sort_by(.key)|map(.value)|flatten(1) )

# Issue:
# - how to fill non-existant cell with null value ?
# - how to get each value in a specific ordered keys list (original unsorted keys)