MM-50024: Handle empty team name for DM/GMs (#22160)

https://mattermost.atlassian.net/browse/MM-50024

```release-note
NONE
```

app/post.go

@@ -2058,8 +2058,16 @@ func (a *App) SetPostReminder(postID, userID string, targetTime int64) *model.Ap
 		return model.NewAppError("SetPostReminder", model.NoTranslation, nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 
-	parsed := time.Unix(targetTime, 0).UTC().Format(time.RFC822)
+	parsedTime := time.Unix(targetTime, 0).UTC().Format(time.RFC822)
 	siteURL := *a.Config().ServiceSettings.SiteURL
+
+	var permalink string
+	if metadata.TeamName == "" {
+		permalink = fmt.Sprintf("%s/pl/%s", siteURL, postID)
+	} else {
+		permalink = fmt.Sprintf("%s/%s/pl/%s", siteURL, metadata.TeamName, postID)
+	}
+
 	// Send an ack message.
 	ephemeralPost := &model.Post{
 		Type:      model.PostTypeEphemeral,
@@ -2070,7 +2078,7 @@ func (a *App) SetPostReminder(postID, userID string, targetTime int64) *model.Ap
 		ChannelId: metadata.ChannelId,
 		// It's okay to keep this non-translated. This is just a fallback.
 		// The webapp will parse the timestamp and show that in user's local timezone.
-		Message: fmt.Sprintf("You will be reminded about %s/%s/pl/%s by @%s at %s", siteURL, metadata.TeamName, postID, metadata.Username, parsed),
+		Message: fmt.Sprintf("You will be reminded about %s by @%s at %s", permalink, metadata.Username, parsedTime),
 		Props: model.StringInterface{
 			"target_time": targetTime,
 			"team_name":   metadata.TeamName,
@@ -2133,7 +2141,7 @@ func (a *App) CheckPostReminders() {
 		for _, postID := range postIDs {
 			metadata, err := a.Srv().Store().Post().GetPostReminderMetadata(postID)
 			if err != nil {
-				mlog.Error("Failed to get post reminder metadata", mlog.Err(err))
+				mlog.Error("Failed to get post reminder metadata", mlog.Err(err), mlog.String("post_id", postID))
 				continue
 			}
 

store/sqlstore/post_store.go

@@ -3313,15 +3313,15 @@ func (s *SqlPostStore) GetPostReminders(now int64) (_ []*model.PostReminder, err
 func (s *SqlPostStore) GetPostReminderMetadata(postID string) (*store.PostReminderMetadata, error) {
 	meta := &store.PostReminderMetadata{}
 	err := s.GetReplicaX().Get(meta, `SELECT c.id as ChannelId,
-			t.name as TeamName,
+		COALESCE(t.name, '') as TeamName,
-			u.locale as UserLocale, u.username as Username
+		u.locale as UserLocale, u.username as Username
-		FROM Posts p, Channels c, Teams t, Users u
+	FROM Posts p
-		WHERE p.ChannelId=c.Id
+	JOIN Channels c ON p.ChannelId=c.Id
-		AND c.TeamId=t.Id
+	LEFT JOIN Teams t ON c.TeamId=t.Id
-		AND p.UserId=u.Id
+	JOIN Users u ON p.UserId=u.Id
-		AND p.Id=?`, postID)
+	AND p.Id=?`, postID)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to get post reminder metadata")
+		return nil, errors.Wrapf(err, "failed to get post reminder metadata: postId %s", postID)
 	}
 
 	return meta, nil

store/storetest/post_store.go

@@ -4657,6 +4657,15 @@ func testGetPostReminderMetadata(t *testing.T, ss store.Store, s SqlStore) {
 	ch, err = ss.Channel().Save(ch, -1)
 	require.NoError(t, err)
 
+	ch2 := &model.Channel{
+		TeamId:      "",
+		DisplayName: "GM_display",
+		Name:        NewTestId(),
+		Type:        model.ChannelTypeGroup,
+	}
+	ch2, err = ss.Channel().Save(ch2, -1)
+	require.NoError(t, err)
+
 	u1 := &model.User{
 		Email:    MakeEmail(),
 		Username: model.NewId(),
@@ -4675,12 +4684,28 @@ func testGetPostReminderMetadata(t *testing.T, ss store.Store, s SqlStore) {
 	p1, err = ss.Post().Save(p1)
 	require.NoError(t, err)
 
+	p2 := &model.Post{
+		UserId:    u1.Id,
+		ChannelId: ch2.Id,
+		Message:   "hi there 2",
+		Type:      model.PostTypeDefault,
+	}
+	p2, err = ss.Post().Save(p2)
+	require.NoError(t, err)
+
 	meta, err := ss.Post().GetPostReminderMetadata(p1.Id)
 	require.NoError(t, err)
 	assert.Equal(t, meta.ChannelId, ch.Id)
 	assert.Equal(t, meta.TeamName, team.Name)
 	assert.Equal(t, meta.Username, u1.Username)
 	assert.Equal(t, meta.UserLocale, u1.Locale)
+
+	meta, err = ss.Post().GetPostReminderMetadata(p2.Id)
+	require.NoError(t, err)
+	assert.Equal(t, meta.ChannelId, ch2.Id)
+	assert.Equal(t, meta.TeamName, "")
+	assert.Equal(t, meta.Username, u1.Username)
+	assert.Equal(t, meta.UserLocale, u1.Locale)
 }
 
 func getPostIds(posts []*model.Post, morePosts ...*model.Post) []string {