[24.0] Fix hasOwner function for histories in client

The `hasOwner` function in `api/index.ts` was checking:
```
return "user_id" in history;
```

whereas, it also needed to check:
```
return "user_id" in history && history.user_id !== null;
```

because we assume histories (summaries) initally fetched for the `historyStore` without user_id are owned by the current user, but the key is still present in the backend return.

This was causing a really troublesome bug in the `HistoryScrollList` found in the history selector modal, where we could constantly keep fetching `api/histories/count` since we didn't have the expected no. of histories loaded in the component; because `historiesProxy` (which only contains owned histories) would end up being empty.

client/src/api/index.ts

@@ -228,5 +228,5 @@ export function userOwnsHistory(user: User | AnonymousUser | null, history: AnyH
 }
 
 function hasOwner(history: AnyHistory): history is HistorySummaryExtended {
-    return "user_id" in history;
+    return "user_id" in history && history.user_id !== null;
 }