Adding new La function named FillNull

[manishkumar1991]

Required items, please complete

Change(s):

• New La function is added to enhances KQL capability to handle Null values inside columns

Reason for Change(s):

• In Sentinel earlier we were not having the capability to replace null values with custom value at one go, so by this function we are intended to bring that functionality

Version Updated:

• Yes

Testing Completed:

• Yes