ExtJS groupTextTpl expression when null

Question

I have a Ext.grid.GroupingView an i group my data with a "field1" but it also can be nul i would like to know if there is a way to change the 'text' on the GroupTextTpl in order to display

Data without Group

When my field1 is null, i tried this

( {gvalue==null?"Null":text} ) ({[values.rs.length]} {[values.rs.length > 1 ? "Items" : "Item"]})

But didn't work, any ideas on this?