MM-38445: Set isFollowing to false only when explicitly unfollowed (#18740)

Summary
Set post.IsFollowing to false only if the post is explicitly unfollowed. Otherwise leave it as nil.

Ticket Link
https://mattermost.atlassian.net/browse/MM-38445

store/sqlstore/post_store.go

@@ -528,7 +528,7 @@ func (s *SqlPostStore) getPostWithCollapsedThreads(id, userID string, extended b
 		"COALESCE(Threads.ReplyCount, 0) as ThreadReplyCount",
 		"COALESCE(Threads.LastReplyAt, 0) as LastReplyAt",
 		"COALESCE(Threads.Participants, '[]') as ThreadParticipants",
-		"COALESCE(ThreadMemberships.Following, false) as IsFollowing",
+		"ThreadMemberships.Following as IsFollowing",
 	)
 	var post postWithExtra
 
@@ -879,7 +879,7 @@ func (s *SqlPostStore) getPostsCollapsedThreads(options model.GetPostsOptions) (
 		"COALESCE(Threads.ReplyCount, 0) as ThreadReplyCount",
 		"COALESCE(Threads.LastReplyAt, 0) as LastReplyAt",
 		"COALESCE(Threads.Participants, '[]') as ThreadParticipants",
-		"COALESCE(ThreadMemberships.Following, false) as IsFollowing",
+		"ThreadMemberships.Following as IsFollowing",
 	)
 	var posts []*postWithExtra
 	offset := options.PerPage * options.Page
@@ -965,7 +965,7 @@ func (s *SqlPostStore) getPostsSinceCollapsedThreads(options model.GetPostsSince
 		"COALESCE(Threads.ReplyCount, 0) as ThreadReplyCount",
 		"COALESCE(Threads.LastReplyAt, 0) as LastReplyAt",
 		"COALESCE(Threads.Participants, '[]') as ThreadParticipants",
-		"COALESCE(ThreadMemberships.Following, false) as IsFollowing",
+		"ThreadMemberships.Following as IsFollowing",
 	)
 	var posts []*postWithExtra
 
@@ -1181,7 +1181,7 @@ func (s *SqlPostStore) getPostsAround(before bool, options model.GetPostsOptions
 			"COALESCE(Threads.ReplyCount, 0) as ThreadReplyCount",
 			"COALESCE(Threads.LastReplyAt, 0) as LastReplyAt",
 			"COALESCE(Threads.Participants, '[]') as ThreadParticipants",
-			"COALESCE(ThreadMemberships.Following, false) as IsFollowing",
+			"ThreadMemberships.Following as IsFollowing",
 		)
 	}
 	query := s.getQueryBuilder().Select(columns...)