offset days null vs zero

Q A TaskDependency created with offset_days null, 0, or the key omitted: what does each read back as? And when a template edge holds 0 and the entity edge null (or the reverse) on the same pair, does the apply of probe 102 keep the edge with its id, or delete and re-create it?

Endpoint POST /entity/task_dependencies ; GET /entity/task_dependencies/<id> ; PUT /entity/task_dependencies/<id> ; PUT /entity/shots/<id> ; POST /entity/task_dependencies/_search

Docs claim Silent. Probe 085 found null and 0 place the dependent Task on the same dates.

Actual

template tasks a..e, edges all finish-to-start-next-day on a
  b on a  sent offset_days=0        POST 201 response offset_days=0         GET offset_days=0
  c on a  sent offset_days=None     POST 201 response has no offset_days    GET offset_days=None
  d on a  sent offset_days=omitted  POST 201 response has no offset_days    GET offset_days=None
  e on a  sent offset_days=0        POST 201 response offset_days=0         GET offset_days=0
  _search offset_days is 0: [b, e]      _search offset_days is None: [c, d]

Shot, PUT task_template T -> 200; edges copied as the template holds them:
  copied            b1 off=0     c1 off=None   d1 off=None   e1 off=0
  PUT b1 offset_days=None, c1 offset_days=0, e1 offset_days=2 -> 200 each
  after edge PUTs   b1 off=None  c1 off=0      d1 off=None   e1 off=2     (same ids)
  PUT Shot task_template=None -> 200, edges unchanged, same ids
  PUT Shot task_template=T    -> 200
                    b1 off=0 NEW id  c1 off=None NEW id  d1 off=None same id  e1 off=0 NEW id
  first ids GET: b1 404, c1 404, d1 200, e1 404
left clean: 0 Tasks, 0 task_dependencies, 0 Shots, 0 TaskTemplates

Teaches

• Null and 0 are two stored values. A create with null or with the key omitted reads back null; a create with 0 reads back 0. offset_days is 0 does not match a null row, and is null does not match a 0.
• The apply compares them as different. Template 0 against entity null, and template null against entity 0, both end with the entity edge deleted (GET 404) and re-created with the template's value and a new id, the same as a real difference (the e1 control, 2 against 0). The unchanged d1 kept its id.
• To keep edge ids through a re-apply, write offset_days exactly as the template holds it, null or 0, never one for the other. Anything keyed on a TaskDependency id loses it otherwise (probe 101: the old row is erased, not retired).
• The POST response omits offset_days when it is null; read it with a GET or a _search.
• Provisioned by the probe; no operator step. Only finish-to-start-next-day was measured, and the dates of null against 0 are probe 085's, not measured again here.