Default to MISSING for GroupPolicies ha timeout

By defaulting to `MISSING`, we make it possible to treat an explicit
`None` as a request for `null` in the payload.

changelog.d/20231025_125551_sirosen_introduce_missingtype.rst

@@ -0,0 +1,5 @@
+Changed
+~~~~~~~
+
+- ``GroupPolicies`` objects now treat an explicit instantiation with
+  ``high_assurance_timeout=None`` as setting the timeout to ``null`` (:pr:`NUMBER`)

src/globus_sdk/services/groups/data.py

@@ -263,7 +263,9 @@ def __init__(
         group_members_visibility: _GROUP_MEMBER_VISIBILITY_T,
         join_requests: bool,
         signup_fields: t.Iterable[_GROUP_REQUIRED_SIGNUP_FIELDS_T],
-        authentication_assurance_timeout: int | None = None,
+        authentication_assurance_timeout: int
+        | None
+        | utils.MissingType = utils.MISSING,
     ):
         super().__init__()
         self["is_high_assurance"] = is_high_assurance
@@ -273,5 +275,5 @@ def __init__(
         )
         self["join_requests"] = join_requests
         self["signup_fields"] = utils.render_enums_for_api(signup_fields)
-        if authentication_assurance_timeout is not None:
+        if authentication_assurance_timeout is not utils.MISSING:
             self["authentication_assurance_timeout"] = authentication_assurance_timeout

tests/unit/helpers/test_groups.py

@@ -0,0 +1,28 @@
+from globus_sdk import GroupPolicies
+
+
+def test_group_policies_can_explicitly_null_ha_timeout():
+    # step 1: confirm that the auth assurance timeout is not in the baseline
+    #         doc which does not specify it
+    kwargs = {
+        "is_high_assurance": True,
+        "group_visibility": "private",
+        "group_members_visibility": "members",
+        "join_requests": False,
+        "signup_fields": [],
+    }
+    baseline = GroupPolicies(**kwargs)
+    assert isinstance(baseline, GroupPolicies)
+    assert "authentication_assurance_timeout" not in baseline
+
+    # step 2: confirm that the auth assurance timeout is in the doc when
+    #         set to an integer value, even 0
+    kwargs["authentication_assurance_timeout"] = 0
+    with_falsy_timeout = GroupPolicies(**kwargs)
+    assert with_falsy_timeout["authentication_assurance_timeout"] == 0
+
+    # step 3: confirm that the auth assurance timeout is in the doc when
+    #         set to None
+    kwargs["authentication_assurance_timeout"] = None
+    with_null_timeout = GroupPolicies(**kwargs)
+    assert with_null_timeout["authentication_assurance_timeout"] is None