[MM-40952] migrations: Add if statements for some queries (#19359)

* migrations: add if statements for some queries

* add better checks to upgrade posts v6 migration

* use "is null" instead of "= null"

* fix down script for users v6

* reflect review comments

db/migrations/postgres/000042_create_threads.up.sql

@@ -9,8 +9,27 @@ ALTER TABLE threads ADD COLUMN IF NOT EXISTS channelid VARCHAR(26);
 
 CREATE INDEX IF NOT EXISTS idx_threads_channel_id ON threads (channelid);
 
-UPDATE threads
-    SET channelId=posts.channelid
-    FROM posts
-    WHERE posts.id=threads.postid
-    AND threads.channelid IS NULL;
+DO $$
+	<< migrate_empty_threads >>
+DECLARE
+	empty_threads_exist boolean := FALSE;
+BEGIN
+	SELECT
+		count(*) != 0 INTO empty_threads_exist
+	FROM
+		threads
+	WHERE
+		channelid IS NULL;
+	IF empty_threads_exist THEN
+		UPDATE
+			threads
+		SET
+			channelId = posts.channelid
+		FROM
+			posts
+		WHERE
+			posts.id = threads.postid
+			AND threads.channelid IS NULL;
+	END IF;
+END migrate_empty_threads
+$$;