[lib] Don't insert username into UserStore when UserInfo missing

Summary:
This addresses [ENG-9373](https://linear.app/comm/issue/ENG-9373/peer-user-bs-app-crashes-when-user-a-deletes-account).

What's happening here:

1. When `findUserIdentities` starts, there is a `UserInfo` for this user in the store
2. By the time `findUserIdentitiesActionTypes.success` is reduced, the `UserInfo` is empty, so we end up inserting just the `username` into the store
3. As a result, we end up passing a `UserInfo` without an ID to C++, which causes a crash

Test Plan:
I tested the following 5 times and confirmed that it didn't cause a crash:

1. Create a test account on iOS Simulator
2. Create a test account on Android
3. Send a friend request from Android to iOS (necessary to create a relationship)
4. Delete account on Android

Reviewers: varun, will

Reviewed By: varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13494

lib/reducers/user-reducer.js

@@ -369,16 +369,19 @@ function reduceUserInfos(
       ];
     }
   } else if (action.type === findUserIdentitiesActionTypes.success) {
-    const newUserInfos = action.payload.userInfos.reduce(
-      (acc, userInfo) => ({
+    const newUserInfos = action.payload.userInfos.reduce((acc, userInfo) => {
+      const existingUserInfo = state.userInfos[userInfo.id];
+      if (!existingUserInfo) {
+        return acc;
+      }
+      return {
         ...acc,
         [userInfo.id]: {
-          ...state.userInfos[userInfo.id],
+          ...existingUserInfo,
           username: userInfo.username,
         },
-      }),
-      {},
-    );
+      };
+    }, {});
 
     const userStoreOps: $ReadOnlyArray<UserStoreOperation> =
       convertUserInfosToReplaceUserOps(newUserInfos);