Various fixes

Fix OAuth and Realtime issues introduced in 9a515b9234

[DATABASE] Fix an empty default value mistake introduced in
fde929b151

[DATABASE][PostgreSQL] Avoid use of pg_constraint.consrc, which was removed in
PostgreSQL 12.

[DATABASE][MariaDB] Fix a typo introduced in aed2344bd4

[DAEMON] Wrap an assignment inside "switch":
a follow-up to adc689cb15

lib/database/schema.php

@@ -878,7 +878,7 @@ class Schema
     {
         // MDB2_Driver_Common::quote changes empty strings to "NULL".
         if ($val === '') {
-            return '';
+            return "''";
         }
         return $this->conn->quote($val);
     }