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

Close #3001
Close #3232
Related: #3244
diff --git a/system/core/Common.php b/system/core/Common.php
index b5a696c..504e225 100644
--- a/system/core/Common.php
+++ b/system/core/Common.php
@@ -289,7 +289,7 @@
 			$_config[0] =& get_config();
 		}
 
-		return isset($_config[0][$item]) ? $_config[0][$item] : FALSE;
+		return isset($_config[0][$item]) ? $_config[0][$item] : NULL;
 	}
 }