fix for issue #1136 Add null support to DataPrepperExpressions - upda…

…ted docs

docs/expression_syntax.md

@@ -83,6 +83,8 @@ Examples
 /status_code == 200 and /message == "Hello world"
 /status_code == 200 or /status_code == 202
 not /status_code in {200, 202}
+/response == null
+/response != null
 ```
 
 # Definitions
@@ -93,6 +95,7 @@ A fundamental value that has no children.
 - Boolean _(Supports true or false)_
 - Json Pointer _(See Json Pointer section for details)_
 - String _(Supports Valid Java String characters)_
+- Null _(Supports null check to see if a Json Pointer is present or not)_
 
 ### Expression String
 The String that will be parsed for evaluation. Expression String is the highest level of a Data Prepper Expression. Only supports one