MM-21645: COALESCE(Bots.LastIconUpdate, 0) (#13638)

Handle a potentially `NULL` Bots.LastIconUpdate and `COALESCE` back to `0`. This column has no default, and while v5.20 should never write it as `NULL`, any pre-v5.20 server writing to the new schema will. As a result, this change ensures backwards compatibility.

Fixes: https://mattermost.atlassian.net/browse/MM-21645
Fixes: https://mattermost.atlassian.net/browse/MM-21585

store/sqlstore/bot_store_test.go

@@ -10,5 +10,5 @@ import (
 )
 
 func TestBotStore(t *testing.T) {
-	StoreTest(t, storetest.TestBotStore)
+	StoreTestWithSqlSupplier(t, storetest.TestBotStore)
 }