Merge pull request yiisoft#12467 from Mak-Di/patch-4

Wrap "null"

docs/guide/input-validation.md

@@ -231,7 +231,7 @@ return [
 ];
 ```
 
-By default, an input is considered empty if its value is an empty string, an empty array or a null.
+By default, an input is considered empty if its value is an empty string, an empty array or a `null`.
 You may customize the default empty detection logic by configuring the [[yii\validators\Validator::isEmpty]] property
 with a PHP callable. For example,