Pick the member branch by list contents so add_members is not treated as a removal

[gpunto]

🔗 Issue Links

No tracking issue.

update_members picks the add/remove branch by whether remove_members is present, but an empty array
is truthy in Ruby:

remove_members = json['remove_members'] ? true : false

• Clients generated from the OpenAPI models send every action list on every call, so an add_members
  request arrives carrying "remove_members":[]. The helper then takes the remove branch, ignores
  add_members, and broadcasts no member.added event.

• The fix picks the branch by whether the list has entries, and defaults member_ids to an empty list so a
  body that omits both keys no longer raises NoMethodError on nil.each.

• Behaviour per payload shape, old vs new:

  body	before	after
  add_members:["jc"], remove_members:[]	REMOVE, []	ADD, ["jc"]
  add_members:[], remove_members:["jc"]	REMOVE, ["jc"]	unchanged
  add_members:[], remove_members:[]	REMOVE, []	ADD, [] (no-op either way)
  add_members:["jc"] (no remove key)	ADD, ["jc"]	unchanged
  remove_members:["jc"] (no add key)	REMOVE, ["jc"]	unchanged
  {}	ADD, nil then raises	ADD, []

  Only the first and last rows change, so bodies from the current clients keep their existing behaviour.

🧪 Testing Notes

Verified the branch selection for each shape above against both the old and the new expression. No E2E test
adds a member from the app today, which is why this has been invisible; the fix is what makes such a test
possible.

Please verify the E2E test runs:

• iOS
  • run 32031158882
• Android
  • run 32031165921
• Flutter
  • run 32031174431