Client want mass change on button click. There are some condition to check on selected records.
Example: If user select 100 records and click button,  it has to check if email is not null  and one more. Both are rollup fields. If any of that 2 fields empty, I want to let the user know and use appropriate view. Because in SF button is available in all list view.If i can make button visible to specific list view, my problem solved. unfortunately thats not that option right now. I need to restrict the user from using other view.
 I used andrew logic in below link and works perfectly .
https://andyinthecloud.com/2015/12/12/visual-flow-with-list-view-and-related-list-buttons/

Here comes the problem, flow couldnt take null value and throwing error. I tried to set the value to "not set"  in visualforce page, but i dont seem working. I tried debug log but in vien.


<apex:page standardController="Contact" tabStyle="Contact" recordSetVar="AllC" >
<!-- Add below each field you reference in your Flow -->
<apex:repeat value="{!AllC}" var="row"  rendered="TRUE">
{!row.Id}
{!IF(ISBLANK(row.Parent_Email__c), 'Not Set',row.Parent_Email__c) } </apex:repeat>
<!-- Runs your Flow -->
<flow:interview name="EmailToContacts" finishLocation="{!$CurrentPage.parameters.retURL}">
 <apex:param name="CVarCon" value="{!Selected}"/> </flow:interview> </apex:page>

This line not working 
{!IF(ISBLANK(row.Parent_Email__c), 'Not Set',row.Parent_Email__c) }

My expectation is: email set to "Not Set" so flow wont throw error and i can also check condition for "Not Set" and warn user to use appropriate view. OR I would like to throw error in visual page itself, that would be perfect.
I am stuck here. Please help.