[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/000051_create_msg_root_count.up.sql

@@ -18,7 +18,7 @@ SELECT count(*) != 0 INTO msg_count_root_exist
     AND column_name = 'mentioncountroot';
 
 IF mention_count_root_exist THEN
-	tmp_count_root := (SELECT count(*) FROM channelmembers WHERE msgcountroot = NULL OR mentioncountroot = NULL);
+	tmp_count_root := (SELECT count(*) FROM channelmembers WHERE msgcountroot IS NULL OR mentioncountroot IS NULL);
 END IF;
 
 ALTER TABLE channelmembers ADD COLUMN IF NOT EXISTS mentioncountroot bigint;