add NULL info

Author: mumian

articles/azure-resource-manager/templates/template-expressions.md

@@ -2,7 +2,7 @@
 title: Template syntax and expressions
 description: Describes the declarative JSON syntax for Azure Resource Manager templates.
 ms.topic: conceptual
-ms.date: 09/03/2019
+ms.date: 02/10/2020
 ---
 
 # Syntax and expressions in Azure Resource Manager templates
@@ -11,6 +11,8 @@ The basic syntax of the template is JSON. However, you can use expressions to ex
 
 A template expression can't exceed 24,576 characters.
 
+Expressions support json('null') and properties support a literal value of null. In both cases, Resource Manager templates treat it as if the property is not present.
+
 ## Use functions
 
 The following example shows an expression in the default value of a parameter: