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/util/daemon.php

@@ -78,12 +78,12 @@ class Daemon
 
     public function background()
     {
-        // Database connection will likely get lost after forking
+        // Database connection will be likely lost after forking.
         $this->resetDb();
 
         // Double-forking.
         foreach (['single', 'double'] as $v) {
-            switch ($pid = pcntl_fork()) {
+            switch (($pid = pcntl_fork())) {
                 case -1: // error
                     common_log(LOG_ERR, 'Could not fork.');
                     return false;