When using XQuery inside SQL Server the value of SQL variables and columns can be referenced using the sql:column and sql:variable functions respectively. When a SQL value is NULL you may want to mark the XML element as xsi:nil, like the following.
<event_data xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true" />
The semantic equivalent of SQL NULL in XQuery is an empty [...]