Convert NULL to Empty String

if ( obj.Item_Description_Code__c == 'ORAS' ){
obj.Item_Description__c = obj.Default_Product_Group_Value__c + '-' + obj.Subcategory_Group_Code__c + ' ' + obj.O_ring_Size_Value__c + ' ' + obj.Material_Family_Value__c + ' ' + obj.Compound_Name_Value__c;
obj.Extended_Description__c = obj.Product_Group_Value__c + ' ' + obj.Subcategory_Group_Value__c + '\n' +
obj.Material_Family_Value__c + ' ' + obj.Durometer__c + ' ' + obj.Color__c + obj.REACH_RoHS_Value__c + obj.Spec_Formula_Value__c + '\n' +
obj.Nominal_ID_Value__c + '"ID x ' + obj.Nominal_CS_Value__c + '"CS Nom. (' + obj.ID__c + '"ID x ' + obj.CS__c + '"CS)' + '\n' +
'Tolerances: AS-568';
        }