chore(issue-details): Don't default to false for streamlined ui (#87337)

with #87336 will allow us to
start opting users into the new experience. Returns `None` instead of
False so that the frontend can determine if we show the new UI or not

Author: roggenkemper

src/sentry/users/api/serializers/user.py

@@ -65,7 +65,7 @@ class _UserOptions(TypedDict):
     defaultIssueEvent: str
     timezone: str
     clock24Hours: bool
-    prefersIssueDetailsStreamlinedUI: bool
+    prefersIssueDetailsStreamlinedUI: bool | None
     prefersSpecializedProjectOverview: dict[str, bool]
     prefersStackedNavigation: bool
     quickStartDisplay: dict[str, int]
@@ -199,7 +199,7 @@ def serialize(
                 "timezone": options.get("timezone") or settings.SENTRY_DEFAULT_TIME_ZONE,
                 "clock24Hours": options.get("clock_24_hours") or False,
                 "prefersIssueDetailsStreamlinedUI": options.get(
-                    "prefers_issue_details_streamlined_ui", False
+                    "prefers_issue_details_streamlined_ui"
                 ),
                 "prefersSpecializedProjectOverview": options.get(
                     "prefers_specialized_project_overview", {}