Fixes #3016.

framework/rbac/DbManager.php

@@ -655,6 +655,9 @@ class DbManager extends Manager
      */
     public function getRule($name)
     {
+        if ($name === null) {
+            return null;
+        }
         $query = new Query;
         $query->select(['data'])->from($this->ruleTable)->where(['name' => $name]);
         $row = $query->createCommand($this->db)->queryOne();

framework/rbac/PhpManager.php

@@ -628,11 +628,7 @@ class PhpManager extends Manager
      */
     public function getRule($name)
     {
-        if (!isset($this->_rules[$name])) {
-            return null;
-        }
-
-        return $this->_rules[$name];
+        return $name !== null && isset($this->_rules[$name]) ? $this->_rules[$name] : null;
     }
 
     /**