MDL-31238 always return object for plugin settings from get_config()

This prevents multiple creations of objects from NULL which is note a warnings in php54

lib/moodlelib.php

@@ -1316,7 +1316,7 @@ function get_config($plugin, $name = NULL) {
         if ($localcfg) {
             return (object)$localcfg;
         } else {
-            return null;
+            return new stdClass();
         }
 
     } 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->assertNull(get_config('filter_name'));
+        $this->assertIdentical(get_config('filter_name'), new stdClass());
     }
 
     public function test_filter_delete_all_for_context() {