Fix panic when parsing empty pgsql host (#28708)

Regression of #27723
Fix #28705

modules/setting/database_test.go

@@ -65,6 +65,10 @@ func Test_getPostgreSQLConnectionString(t *testing.T) {
 		SSLMode string
 		Output  string
 	}{
+		{
+			Host:   "", // empty means default
+			Output: "postgres://:@127.0.0.1:5432?sslmode=",
+		},
 		{
 			Host:    "/tmp/pg.sock",
 			User:    "testuser",