Add null to iff (#461)

Author: manototh

apl/scalar-functions/conditional-function.mdx

@@ -9,10 +9,10 @@ keywords: ['axiom documentation', 'documentation', 'axiom', 'case', 'iff']
 
 | **Function Name**   | **Description**                                                                                      |
 | ----------------- |  ------------------------------------------------------------------------------------------------- |
-| [case()](#case) | Evaluates a list of conditions and returns the first result expression whose condition is satisfied. |
-| [iff()](#iff) | Evaluates the first argument (the predicate), and returns the value of either the second or third arguments |
+| [case](#case) | Evaluates a list of conditions and returns the first result expression whose condition is satisfied. |
+| [iff](#iff) | Evaluates the first argument (the predicate), and returns the value of either the second or third arguments |
 
-## case()
+## case
 
 Evaluates a list of conditions and returns the first result whose condition is satisfied.
 
@@ -49,7 +49,7 @@ extend status_human_readable = case(
 
 [Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'sample-http-logs'%5D%5Cn%7C%20extend%20status_code%20%3D%20case(status_int%20%3D%3D%20200%2C%20'OK'%2C%20status_int%20%3D%3D%20201%2C%20'Created'%2C%20status_int%20%3D%3D%20301%2C%20'Moved%20Permanently'%2C%20status_int%20%3D%3D%20500%2C%20'Internal%20Server%20Error'%2C%20'Other')%22%7D)
 
-## iff()
+## iff
 
 Evaluates the first argument (the predicate), and returns the value of either the second or third arguments. The second and third arguments must be of the same type.
 
@@ -76,4 +76,12 @@ iff(predicate, ifTrue, ifFalse)
 | project Status = iff(req_duration_ms == 1, "numeric", "Inactive")
 ```
 
-[Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'sample-http-logs'%5D%5Cn%7C%20project%20Status%20%3D%20iff%28req_duration_ms%20%3D%3D%201%2C%20%5C%22numeric%5C%22%2C%20%5C%22Inactive%5C%22%29%22%7D)
+[Run in Playground](https://play.axiom.co/axiom-play-qf1k/query?initForm=%7B%22apl%22%3A%22%5B'sample-http-logs'%5D%5Cn%7C%20project%20Status%20%3D%20iff%28req_duration_ms%20%3D%3D%201%2C%20%5C%22numeric%5C%22%2C%20%5C%22Inactive%5C%22%29%22%7D)
+
+<Note>
+To return a null value from `iff`, use `dynamic(null)`.
+
+```kusto
+iff(condition, dynamic(null), value)
+```
+</Note>