config_item() to return NULL instead of FALSE for non-existing items

Close #3001
Close #3232
Related: #3244
diff --git a/tests/mocks/core/common.php b/tests/mocks/core/common.php
index 5c32ca5..2e8265b 100644
--- a/tests/mocks/core/common.php
+++ b/tests/mocks/core/common.php
@@ -32,7 +32,7 @@
 
 		if ( ! isset($config[$item]))
 		{
-			return FALSE;
+			return NULL;
 		}
 
 		return $config[$item];