regression fix for: #2357 scrolling with large amount of columns,columns
disappear after sort 

scrollTop can now be undefined/null then that value should not be used
for calculating if this is a scroll that doesn't have to update
anything.
Merge pull request #2379 from Servoy/row_inserts_before_current_fix

regression fix for: #2357 scrolling with large amount of columns,columns disappear after sort