MDL-31006 revert partially @ get_config()

The change from null to stdClass() in get_config() was leading to:

1) unit tests not passing.
2) non-equivalent evaluation in conditions (null evals false,
   stdClassi() evals true)

lib/moodlelib.php

@@ -1318,7 +1318,7 @@ function get_config($plugin, $name = NULL) {
         if ($localcfg) {
             return (object)$localcfg;
         } else {
-            return new stdClass();
+            return null;
         }
 
     } else {

lib/simpletest/testfilterconfig.php

@@ -747,7 +747,7 @@ class filter_delete_config_test extends UnitTestCaseUsingDatabase {
         $expectedconfig = new stdClass;
         $expectedconfig->configname = 'Other config value';
         $this->assertEqual($expectedconfig, get_config('filter_other'));
-        $this->assertFalse(get_config('filter_name'));
+        $this->assertNull(get_config('filter_name'));
     }
 
     public function test_filter_delete_all_for_context() {