Database: Initialize xorm with an empty schema for postgres (#17357)

xorm introduced some changes in
https://github.com/go-xorm/xorm/pull/824 and
https://github.com/go-xorm/xorm/pull/876 which by default will use
public as the postgres schema and this was a breaking change compared
to before. Grafana has implemented a custom postgres dialect so above
changes wasn't a problem here. However, Grafana's custom database
migration was using xorm dialect to check if the migration table exists
or not.
For those using a custom search_path (schema) in postgres configured on
server, database or user level the migration table check would not find
the migration table since it was looking in public schema due to xorm
changes above. This had the consequence that Grafana's database
migration failed the second time since migration had already run
migrations in another schema.
This change will make xorm use an empty default schema for postgres and
by that mimic the functionality of how it was functioning before
xorm's changes above.
Fixes #16720

Co-Authored-By: Carl Bergquist <carl@grafana.com>

pkg/services/sqlstore/sqlstore.go

@@ -37,6 +37,11 @@ var (
 const ContextSessionName = "db-session"
 
 func init() {
+	// This change will make xorm use an empty default schema for postgres and
+	// by that mimic the functionality of how it was functioning before
+	// xorm's changes above.
+	xorm.DefaultPostgresSchema = ""
+
 	registry.Register(&registry.Descriptor{
 		Name:         "SqlStore",
 		Instance:     &SqlStore{},