title: How to check if variable empty in Drools Templates
body: <p>I'm using Drools templates to generate Rules from a spreadsheet.
  I have some problems with empty cell. 
  Does anybody know how to check if a cell or the variable is empty?
  This is the template:</p>
  
  <pre><code>template header
  pid
  department
  maxAmount
  storeId
  
  package X.X.X
  
  declare MaxAmountStore
      amount : Integer    
      storeId : String
  end
  
  template "FOO"
  rule "MaxAmountStore_@{row.rowNumber}"
          when
              Product(pid == "@{pid}")
              Product(departmentId == "@{department}")
          then
              System.out.println("New MaxAmountStore: @{maxAmount} (store: @{storeId})");
              insert( new MaxAmountStore(@{maxAmount}, "@{storeId}"));
      end
  end
  </code></pre>
  
  <p>Is it possible to have an empty storeId?</p>