Get posts for a channel has_next attribute fix (#26901)

* Get posts for a channel has_next attribute fix

* HasNext attribute test changes

* tests fix

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/post_list.go

@@ -15,7 +15,7 @@ type PostList struct {
 	NextPostId string           `json:"next_post_id"`
 	PrevPostId string           `json:"prev_post_id"`
 	// HasNext indicates whether there are more items to be fetched or not.
-	HasNext bool `json:"has_next"`
+	HasNext *bool `json:"has_next,omitempty"`
 	// If there are inaccessible posts, FirstInaccessiblePostTime is the time of the latest inaccessible post
 	FirstInaccessiblePostTime int64 `json:"first_inaccessible_post_time"`
 }