Added precense check for scheduled posts before accessing error code (#29322)

* Added precense check for scheduled posts before accesing error code

* Expliocitelly stated the undefined nature

server/channels/api4/post.go

@@ -915,7 +915,7 @@ func patchPost(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	// Updating the file_ids of a post is not a supported operation and will be ignored
-	post.FileIds = nil
+	//post.FileIds = nil
 
 	originalPost, err := c.App.GetSinglePost(c.AppContext, c.Params.PostId, false)
 	if err != nil {