Coalesce migration of Reactions.ChannelId to '' if Post does not exist. (#20643)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

db/migrations/mysql/000089_add-channelid-to-reaction.up.sql

@@ -14,7 +14,7 @@ EXECUTE addColumnIfNotExists;
 DEALLOCATE PREPARE addColumnIfNotExists;
 
 
-UPDATE Reactions SET ChannelId = (select ChannelId from Posts where Posts.Id = Reactions.PostId) WHERE ChannelId="";
+UPDATE Reactions SET ChannelId = COALESCE((select ChannelId from Posts where Posts.Id = Reactions.PostId), '') WHERE ChannelId="";
 
 
 SET @preparedStatement = (SELECT IF(

db/migrations/postgres/000089_add-channelid-to-reaction.up.sql

@@ -1,3 +1,3 @@
 ALTER TABLE reactions ADD COLUMN IF NOT EXISTS channelid varchar(26) NOT NULL DEFAULT '';
-UPDATE reactions SET channelid = (select channelid from posts where posts.id = reactions.postid) WHERE channelid='';
+UPDATE reactions SET channelid = COALESCE((select channelid from posts where posts.id = reactions.postid), '') WHERE channelid='';
 CREATE INDEX IF NOT EXISTS idx_reactions_channel_id on reactions (channelid);