Changing the SPARK encoding of empty non-primitive elements in HAPI objects

[piotrszul]

Changing the SPARK encoding of empty non-primitive elements in HAPI objects to produce NULL values rather than empty structs/arrays.

For example the encoding of:

"identifier" : [
  {
    "use" : "official",
    "system" : "https://github.com/synthetichealth/synthea",
    "value" : "d2120f2d-bc32-096d-d854-1a1e73a05f0d"
  }
],

produce the row:

Row(identifier=[Row(id=None, use='official', type=None, system='
[https://github.com/synthetichealth/synthea'](https://github.com/synthetichealth/synthea%27)
, value='e4fc9c8b-66b7-7f4b-a033-39acfc7633fa', period=None, assigner=None)])

where composite elements such aas period or assigner as set to NULL (None).

The same also applies to empty elements with many cardinality. They produce NULL rather then an empty array().