[SalesForce] Why I can’t change OwnerId to null or empty

Since this hasn't been responded to after having been posted for several weeks, it would appear that no one has a timely suggestion for you.

My suggestion would be pre-process your CSV files using a script that scans them for field entries containing blank values and inserts a "#N/A" if it finds one. You could do that separately or as part of some kind of "front end" processing by your Utility Tool. You may have some other way of doing that via Java, but I'd think a simple Bat file or VB Script would do the job for you without requiring much work on your part to create it.

I am happy that my comment above has helped you to solve your most burning problem. Let me please mention another problem with your code. As the trigger could potentially be called for up to 200 records at once, you need to move the SOQL query out of the loop. Otherwise you might be getting a too many SOQL queries exception - and possibly some performance issues.

Assuming that you only want the trigger to run after update, I suggest you try this code (not tested, please feed back if it works or not):

trigger AsessmentUpdateOpp on PlatformCase__c (after update) {

    Set<Id> oportunityIds = new Set<Id>();
    for (PlatformCase__c c : Trigger.new) {
        oportunityIds.add(c.Opportunity__c);
    }

    List<Opportunity> opps : [SELECT Id, Building_Sq_Footage__c 
            FROM Opportunity 
            WHERE Id in :oportunityIds ]);

    for (Opportunity opp: opps) {
        opp.Building_Sq_Footage__c += c.Building_Sq_Footage__c; 
    }

    update opps;

}

You might also want to think about update and delete triggers, I could imagine they'd be important to keep the data correct. Also, you might want to read up on Roll-Up Summary fields