Includes its body, if the user check validates.
----
!Usage
<wiki:UserCheck exists="true|false">
;:''JSP or HTML content''
</wiki:UserCheck>
!Parameters
;__exists__: If set to "true", then the content is included if there is a valid username; if set to "false", includes the content when there is no valid username.
!Example
{{{
<wiki:UserCheck exists="true">
G'day,<wiki:UserName />
</wiki:UserCheck>
<wiki:UserCheck exists="false">
Set your name in <wiki:LinkTo page="UserPreferences">UserPreferences</wiki:LinkTo>
</wiki:UserCheck>
}}}
Displays a greeting, if the user has validated; else displays a link to the page UserPreferences.